Behavior Of Point Defects Research Articles

Differing behaviors of point defects in Fe–36Ni alloy during annealing after low-temperature deformation and irradiation

Fe–36Ni alloy is studied in states in which it is quenched, irradiated with electrons at 70 K, and deformed at 77 K. It is shown that similar structure-phase transformations occur in the investigated alloy during annealing at the temperature range of 70–570 K both after preliminary low-temperature electron irradiation and after preliminary low-temperature deformation. Migration of interstitial atoms (70–140 K) and vacancies (180–570 K), which induce phase transformations and are the results of atomic separation and ordering of zones with increased nickel content, is observed. Mechanisms of structure-phase changes under deformation and annealing are discussed.

Manipulation of native point defect behavior in rutile TiO2 via surfaces and extended defects

Semiconductor surfaces offer efficient pathways for exchanging native point defects with the underlying bulk. For rutile TiO2(1 1 0), isotopic self-diffusion studies of oxygen have suggested that the surface may act as a source for Oi while simultaneously acting as a sink for titanium interstitials Tii. Through self-diffusion measurements with labeled Ti as well as O, the present work develops a more complete picture of the diffusion-reaction network involving Oi and Tii, complete with the surface acting as a source for whichever elements are available from the gas phase and a sink for elements that are not. The picture points to the importance of extended defects such as platelets and crystallographic shear planes as reservoirs of Oi and Tii, acting as net sources or sinks of these species depending upon specific conditions. The results exemplify the combined roles of surfaces and extended defects in regulating point defect behavior even in macroscopic metal oxide crystals, and point to specific strategies for manipulating that behavior intentionally.

About the influence of deposited nitride layers on oxide precipitation after RTA pre‐treatment

In order to elucidate the influence of stress and in‐diffused nitrogen on oxide precipitation after rapid thermal annealing (RTA), we carried out experiments with one‐side and double‐sided nitride layers accompanied by simulation models which help to understand the behavior of intrinsic point defects, nitrogen, and strain. We found that the presence of a nitride layer of any thickness, within the rage which we investigated, in direct contact with the silicon surface is sufficient to markedly change the precipitation behavior of interstitial oxygen after RTA at 1175 and 1250 °C. The presence of the nitride layer during the stabilization and growth of the oxide precipitates is not of any influence on the precipitation behavior. Therefore, the RTA of wafers covered with silicon nitride is the crucial step controlling the bulk microdefect (BMD) depth profiles. A 10 nm oxide between silicon substrate and nitride layer prevents any change of the BMD depth profile. Only in a direct contact with the nitride layers the vacancy supersaturation, which enhanced the oxide precipitation compared to wafers without nitride layers, was generated. Nitrogen peaks below the silicon surface generated by in‐diffusion of nitrogen during RTA lead to an enhanced oxygen precipitation only for RTA at 1250 °C and not for RTA at 1175 °C. We propose a model based on very tiny coherent α‐Si3N4 precipitates generated at nitrogen‐vacancy (NV) complexes which can act as nucleation sites for oxygen precipitation. Because the stability of NV seems limited to temperatures above 1200 °C, it would not be effective for RTA at 1175 °C. RTA treatment of silicon wafers with one‐sided nitride layers at 1250 °C leads to very sharp and small defect denuded zones in subsequent annealing and would be suitable for proximity gettering. The depth of the denuded zone is nearly independent of the thickness of the nitride layer.

Accumulation of dislocation loops in the α phase of Zr Excel alloy under heavy ion irradiation

In-situ heavy ion irradiations were performed on the high Sn content Zr alloy ‘Excel’, measuring <a> type dislocation loop accumulation up to irradiation damage doses of 10 dpa at a range of temperatures. The high content of Sn, which diffuses slowly, and the thin foil geometry of the sample provide a unique opportunity to study an extreme case where displacement cascades dominate the loop formation and evolution. The dynamic observation of dislocation loop evolution under irradiation at 200 °C reveals that <a> type dislocation loops can form at very low dose (0.0025 dpa). The size of the dislocation loops increases slightly with irradiation damage dose. The mechanism controlling loop growth in this study is different from that in neutron irradiation; in this study, larger dislocation loops can condense directly from the interaction of displacement cascades and the high concentration of point defects in the matrix. The size of the dislocation loop is dependent on the point defect concentration in the matrix. A negative correlation between the irradiation temperature and the dislocation loop size was observed. A comparison between cascade dominated loop evolution (this study), diffusion dominated loop evolution (electron irradiation) and neutron irradiation suggests that heavy ion irradiation alone may not be enough to accurately reproduce neutron irradiation induced loop structures. An alternative method is proposed in this paper. The effects of Sn on the displacement cascades, defect yield, and the diffusion behavior of point defects are established.

Asymmetric interaction of point defects and heterophase interfaces in ZrN/TaN multilayered nanofilms

Materials with a high density of heterophase interfaces, which are capable of absorbing and annihilating radiation-induced point defects, can exhibit a superior radiation tolerance. In this paper, we investigated the interaction behaviors of point defects and heterophase interfaces by implanting helium atoms into the ZrN/TaN multilayered nanofilms. It was found that the point defect-interface interaction on the two sides of the ZrN/TaN interface was asymmetric, likely due to the difference in the vacancy formation energies of ZrN and TaN. The helium bubbles could migrate from the ZrN layers into the TaN layers through the heterophase interfaces, resulting in a better crystallinity of the ZrN layers and a complete amorphization of the TaN layers. The findings provided some clues to the fundamental behaviors of point defects near the heterophase interfaces, which make us re-examine the design rules of advanced radiation-tolerant materials.

Theoretical Study of Impact of Internal and External Stresses on Thermal Equilibrium Concentrations of Intrinsic Point Defects in Doped Si Crystals

Intrinsic point defects (vacancy V and self-interstitial I) play an important role in a wide variety of processes in Si crystals. During large-diameter crystal growth, the thermal stress, which is the compressive internal stress, of around several tens of MPa is originated in Si crystals. In the LSI (Large Scale Integration) using Si substrate, external stress of around GPa order is applied to the channel region between source and drain in the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) to enhance the carrier mobilities. These internal and external stresses affect V and I; thus, changing the thermal equilibrium concentrations (Ceq) of V and I in Si. To explain and engineer the intrinsic point defect behavior in Si crystals, such as in the growing Si crystals and in the channel region of Si wafers, the impact of internal or external stress on the Ceq of V (I) should be theoretically obtained in doped Si crystals. In this study, we calculated the formation energy (Ef) and relaxation volume (ΔVol) of V (I) from the 1st to 5th sites around dopant (B, C, Sn, P, and Sb) atoms in isotropically stressed Si by using density functional theory (DFT) to determine the formation enthalpy (Hf) of V and I. The impact of isotropic internal stress (σin) or isotropic external stress (σex) on the Ceq values of V (I) in doped Si was obtained using the Hf at each site around a dopant atom. The following important information was obtained from the Ceq of V (I) in doped Si crystals. (1) The Ef and ΔVol of V (I) around the dopant atoms change depending on the site position and dopant type, (2) Ef of V (I) and ΔVol of I around the dopant atoms are hardly affected by the stresses, while ΔVol of V around the dopant atoms changes due to the stresses, (3) B and C atoms make the Si crystal more I-rich, while Sn, P, and Sb atoms make the Si crystal more V-rich, and (4) the compressive σin makes the doped Si crystal more V-rich, and its impact depends on the type of dopant, while the σex has an opposite impact except for C. The data summarizing the impact of stress in doped Si are useful as the more precisely control of the point defect behavior is required in more advanced Si crystals.

The structures and diffusion behaviors of point defects and their influences on the electronic properties of 2D stanene

During the synthesis of stanene monolayers, defects are inevitably present and always affect the properties.

Theoretical study of the impact of stress and interstitial oxygen on the behavior of intrinsic point defects in growing Czochralski Si crystals

For the development of crystal pulling processes for 450mm-diameter defect-free Si crystals, it is important to evaluate the impact of thermal stress on intrinsic point defect behavior during crystal growth. In a crystal growing from a melt, the melt/solid interface can be considered as being stress-free. Due to that the thermal stress in the growing substrate near the interface is internal plane stress. Previously, we evaluated the impact of (001) planar-isotropic stress on the formation enthalpy (Hf) of the vacancy (V) and the self-interstitial (I) using density functional theory (DFT) calculations, and explained quantitatively the published experimental values of the so-called “Voronkov criterion”. The thermal stress in a growing crystal is indeed planar but is not isotropic in the plane except for the central region of the crystal. The purpose of the present study is to estimate the impact of planar-anisotropic stress on the formation enthalpy Hf of V and I. It is found that the three stress dependencies of σx: σy=1: 1 (planar-isotropic), 2: 1, 5: 1 (planar-anisotropic) are close to each other, independent of the assumption of isotropic or anisotropic planar stress. This is the reason why the experimental results obtained over the whole radial direction of the crystal are well reproduced by the calculated results assuming planar-isotropic stress. A uniaxial stress dependence which is a good assumption for the crystal peripheral region, leads also to results that are close to those for the planar stress dependence. Also the mechanisms behind the experimentally observed impact of interstitial oxygen (Oi), introduced during Czochralski Si growth, on V and I concentrations are clarified. DFT calculations are performed to obtain the formation energies (Ef) of V and I at all sites within a sphere with 5Å radius around the Oi atom. Formation (vibration) entropy (Sf) calculations for V and I are also performed. It is found that both EfV and SfV of V in the zigzag-bond (1st, 2nd, 5th) including the Oi atom decrease while EfI of I is not affected by the Oi atom. “Total V” is defined as the sum of free V and V trapped by the Oi atoms. The total V concentration at the melting point is evaluated by considering the EfV and SfV at each site. The calculated V concentration increases by about 2.9% with 1×1018 Oicm−3 and agrees well with the experimentally estimated value of a few % increase with 1×1018 Oicm−3.

Thermal equilibrium concentration of intrinsic point defects in heavily doped silicon crystals - Theoretical study of formation energy and formation entropy in area of influence of dopant atoms-

It is well known that p-type, neutral and n-type dopants affect the intrinsic point defect (vacancy V and self-interstitial I) behavior in single crystal Si. By the interaction with V and/or I, (1) growing Si crystals become more V- or I-rich, (2) oxygen precipitation is enhanced or retarded, and (3) dopant diffusion is enhanced or retarded, depending on the type and concentration of dopant atoms. Since these interactions affect a wide range of Si properties ranging from as-grown crystal quality to LSI performance, numerical simulations are used to predict and to control the behavior of both dopant atoms and intrinsic point defects. In most cases, the thermal equilibrium concentrations of dopant-point defect pairs are evaluated using the mass action law by taking only the binding energy of closest pair to each other into account. The impacts of dopant atoms on the formation of V and I more distant than 1st neighbor and on the change of formation entropy are usually neglected. In this study, we have evaluated the thermal equilibrium concentrations of intrinsic point defects in heavily doped Si crystals. Density functional theory (DFT) calculations were performed to obtain the formation energy (Ef) of the uncharged V and I at all sites in a 64-atom supercell around a substitutional p-type (B, Ga, In, and Tl), neutral (C, Ge, and Sn) and n-type (P, As, and Sb) dopant atom. The formation (vibration) entropies (Sf) of free I, V and I, V at 1st neighboring site from B, C, Sn, P and As atoms were also calculated with the linear response method. The dependences of the thermal equilibrium concentrations of trapped and total intrinsic point defects (sum of free I or V and I or V trapped with dopant atoms) on the concentrations of B, C, Sn, P and As in Si were obtained. Furthermore, the present evaluations well explain the experimental results of the so-called “Voronkov criterion” in B and C doped Si, and also the observed dopant dependent void sizes in P and As doped Si crystals. The expressions obtained in the present work are very useful for the numerical simulation of grown-in defect behavior, oxygen precipitation and dopant diffusion in heavily doped Si. DFT calculations also showed that Coulomb interaction reaches approximately 30Å from p (n)-type dopant atoms to I (V) in Si.

Study of Radiation Effects on Er3+-Doped Nanoparticles Germano-Silica Fibers

The main goal of the present work is to study the impact of Er–SiO2 and Er–Al2O 3 nanoparticles on the radiation induced defects in germano-silica optical fibers with Al/Ge ranges over 0.1–150. These fibers are prepared by a modified chemical vapor deposition technique and are characterized by optical absorption and electron paramagnetic resonance spectroscopy. High amount of Ge, Er–SiO2 nanoparticles-derived optical fibers exhibit lower radiation induced attenuation than those with a low Ge content undoped or doped with Al2O3 nanoparticles. Furthermore, the behavior of point defects namely GeE’, Ge(1), SiE’, nonbridging oxygen hole centers, and H(1) is reported according to the Al/Ge ratio. In contrast to the optical fiber preforms, no Al-related defects are found in the optical fibers. Therefore, the results evidence the strong radiation tolerance by the virtue of the nanoparticle doping, which is enabling technology for the development of the photonic and space applications in the radiation fields.

Atomic-scale investigation of triple junction role on defects binding energetics and structural stability in α-Fe

Nanocrystalline (NC) metals (mean grain sizes d ≤ 100 nm) have enhanced mechanical strength as compared to coarse-grained metals (d ≥ 1 μm), and thus, are a promising alternative as structural materials for future high energy nuclear reactors. However, during extreme conditions, the NC microstructure has been found to be thermodynamically unstable, thereby limiting its applicability. Further, for materials with average grain size <10 nm, the triple junctions (TJs) have been observed to have a significant contribution on the mechanical behavior and microstructural stability. Moreover, at the atomic-scale, the region surrounding the TJ demonstrates unique physical properties, such as rapid diffusion, non-equilibrium segregation and increased dislocation activity. Therefore, in this work, we systematically assess the role of TJs on the structural stability and the solute binding behavior in α-Fe. Using atomistic simulations, we show that the TJ resolved line tension is strongly correlated (inversely) with the mean activation energy for self-diffusion along the TJ, i.e., the thermodynamically unstable TJs demonstrated a lower activation energy barrier for self-diffusion along TJs with higher line tension. Next, we demonstrated that the strain energy evolution around the TJ can provide insights into the distinct binding behavior of point defects and solute atoms. In other words, the examination of solute binding behavior revealed a localized region of stable sites around the TJs which aids in accommodation of high solute concentration at high temperatures. In summary, our findings quantify the distinct role of TJs on the defect (vacancy, self-interstitial and solute atom) binding and migration behavior and these findings are necessary for designing future structural materials for extreme environments, including those needed in aerospace, naval, civilian and energy sector infrastructures.

Ab initio study of tungsten defects near the surface

A first principles analysis of the behaviour of point defects, namely, self-interstitial atoms, a single vacancy and light impurity atoms such as H and He in tungsten is reported. These defects can be produced in the first wall of the future nuclear fusion reactors due to the high radiation fluxes present. The evolution of defects that appear in the bulk and end up reaching the surface has been followed. An energetic study has been combined with a detailed charge density analysis of the system by means of the SIESTA code. The resulting data have been validated by confronting them with those obtained with a more precise plane wave code, namely VASP. Meanwhile, the structural and the mechanical properties of the system have been positively compared with experimental measurements. Such comparisons have led us to present a new SIESTA basis for tungsten. This complete analysis establishes a nanoscopic view of the phenomena involving the presence of light atoms at native defects in tungsten, paying special attention to the vicinity of surfaces.

First-principles study on mono-vacancy self diffusion and recovery in tungsten crystal

The point defects behavior becomes one of the most basic issues under the challenge of fusion environment. The recovery mechanisms of Frenkel pair defects and the self-diffusion coefficient of mono-vacancy in bulk bcc tungsten were researched by the first principle calculations. The calculation of migration energy curves for <111> SIAs indicated that the process of the Frenkel pair recovery had a close correlation with the “I–V” distance, and the migration barrier energies Em was within a limit range of 1.86–2.08eV. It was found that the self-recovery region had an ellipsoid profile with the semiminor axis of 2.7Å and the semimajor axis of 5.5Å. The self-diffusion coefficients of the mono-vacancy were calculated and the results showed that the probability for the vacancy migration was not only associated with the Em but also the temperature being challenged.

Impact of Anisotropic Thermal Stress on Behavior of Grown-In Defects during Si Crystal Growth from a Melt

Anisotropic thermal stress near the melt/solid interface during growth of a Si crystal from a melt significantly affects control of grown-in defects when the diameter of the growing crystal is large. This effect on the behavior of intrinsic point defects in a growing Si crystal with 300-mm diameter was investigated by calculations and experiments. One has to take into account this effect in the development of a crystal-pulling process for 450-mm-diameter defect-free Si crystals.

シリコン単結晶育成中の点欠陥挙動に与える置換型ドーパントと熱応力の効果

The mechanisms behind the experimentally observed impact of the type and concentration of substitutional dopants on intrinsic point defect behavior in growing single crystal Si are clarified. On the basis of the density functional theory (DFT) calculated results, an appropriate model of intrinsic point defect behavior in heavily doped Si is proposed. Also one has to take into account the impact of thermal stress on intrinsic point defect behavior during single crystal Si growth. In order to explain the experimental results quantitatively, the dependence of the formation enthalpies of vacancy (V) and self-interstitial (I) on compressive plane stress was determined using DFT calculations. It is found that the compressive plane stress around 20 MPa shifts a growing Si crystal to more V-rich. The calculated plane stress dependence is in excellent agreement with the published experimental values and should be taken into account in the development of pulling processes for 450 mm diameter defect-free Si crystals.

Oxygen self-diffusion in polycrystalline uranium–plutonium mixed oxide U0.55Pu0.45O2

Atomic transport properties in uranium–plutonium mixed oxides U1-yPuyO2 are of essential concern because they impact numerous aspects of their physicochemical behavior at all stages of the fuel cycle. In this paper, we report oxygen tracer diffusion coefficients in homogeneous U0.55Pu0.45O2 mixed oxide. The study is based on tracer diffusion coefficient measurements obtained using Secondary Ion Mass Spectrometry (SIMS) following diffusion annealing involving gas–solid 18O/16O isotopic exchange. As for other fundamental material properties governed by the nature and behavior of point defects, we show that a careful study of tracer diffusion coefficients as a function of oxygen partial pressure and temperature is liable to provide insight into prevailing diffusion mechanisms. Under the conditions studied in this work, it appears that oxygen diffusion is vacancy mediated.

Effects of atomic grain boundary structures on primary radiation damage in α-Fe

In this paper, we used five different grain boundary (GB) structures including two twists, two symmetric tilts and one asymmetric tilt for α-Fe, in order to study the influence of different GB structures on the production and time evolution of defects. Energetic behavior of point defects near GBs is investigated and analyzed using Molecular Static (MS) method to calculate and compare the “defect absorbency” of each structure. The primary radiation damage state near each GB structure is simulated using Molecular Dynamic (MD) method for 3keV and 6keV primary-knocked on atom (PKA) with velocity vector perpendicular to the GB plane at various distances in 300K. We found that all five GB structures can decrease the defect number in bulk region, if cascade center locates on the GB plane (prefect overlap) and increase the vacancy number, if the overlap is imperfect. This depends on the energy of PKA and its distance from GB plane. Also, the results proved that the magnitude of the observed variations depends on the atomic structure of GB. Furthermore, the GBs that have stronger “interstitial absorbency” produce an excess concentration of vacancies in the bulk region, while the edge of the cascades overlaps with GB plane. This is the result of bigger “interstitial absorbency” of GBs in comparison with vacancies.

Describing oxygen self-diffusion in PuO2 by connecting point defect parameters with bulk properties

The description of oxygen self-diffusion over a range of temperatures and pressures is important in PuO2 for nuclear fuel applications. Although there are limited experimental studies describing oxygen self-diffusion in PuO2, recent molecular dynamics studies extend the temperature range significantly. In the present study elastic and expansivity data is used in the framework of a thermodynamic model (known as the cBΩ model) to derive the oxygen self-diffusion coefficient in PuO2 in the temperature range 1800–3000 K. In the cBΩ model the defect Gibbs energy is proportional to the isothermal bulk modulus (B) and the mean volume per atom (Ω). The derived results are in good agreement with the most recent experimental and molecular dynamics data. Importantly, the present study extends the applicability of the model to nuclear fuel materials for the first time, where point defect parameters and behaviour are difficult to determine, particularly at the temperatures considered here.

In situ UHVEM irradiation study of intrinsic point defect behavior in Si nanowire structures

AbstractSi nanowire‐based Tunnel‐Field Effect Transistor (TFET) characteristics are intensively studied as function of nanowire diameter and doping. A significant reduction of B diffusion with decreasing nanowire diameter is e.g. observed and attributed to reduced transient enhanced diffusion close to the nanowire surface caused by the recombination and out‐diffusion of excess self‐interstitials. In an Ultra High Voltage Electron Microscope (UHVEM), the formation of self‐interstitial clusters can be studied in situ while varying e‐beam flux, irradiation temperature, impurity concentration and capping layers surrounding the nanowires.Results are presented on {113}‐defect formation in Si nanowires with diameters between 40 and 500 nm. The Si nanowires are embedded in SiO2 and are etched into an epitaxial Si stack on a heavily As doped Si substrate. The top layer of the epitaxial stack is in situ B doped or B implanted. In situ UHVEM studies are performed on focused ion beam prepared cross‐section samples, irradiating with different fluxes of 2 MeV electrons between room temperature and 375 °C. A strong dependence of {113}‐defect formation on nanowire radius and doping is observed. The observations are compared with simulations based on quasi‐chemical reaction rate theory. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Zinc vacancy related emission in homoepitaxial N-doped ZnO microrods

Vertically aligned N-doped ZnO microrods have been homoepitaxially grown on a high-quality ZnO template via chemical vapor transport method without employing any catalysts. In spite of the incorporation of nitrogen, the as-grown microrods exhibited excellent crystalline and optical quality, and the behavior of intrinsic point defects and impurity-related defects have been investigated in detail by the temperature-dependent photoluminescence spectra. The emissions at 3.377, 3.368, and 3.363eV are ascribed to free excitons, surface bound excitons and excitons bound to neutral shallow donors: zinc interstitials, respectively. The emissions at 3.359, 3.311, and 3.240eV correspond to the radiative recombination of excitons bound to neutral acceptors, free electrons to acceptors and donor acceptor pairs, all of which are originated from the same shallow acceptor of zinc vacancy clusters with its energy level of about 126meV above the valence band maximum. The isolated zinc vacancy is a deep acceptor, which results in the emission at 3.318eV and the green band emissions. This study suggests that the intrinsic point defects play an extremely important role in optical properties of ZnO besides the external dopants and the understanding of intrinsic defects is crucial to get stable p-type conductivity and finally realize a ZnO homojunction for optoelectronic applications.