Dominant Point Defects Research Articles

Point defects in active layers of TFEL devices based on ZnS

Point defects in the active layer of a layered optimization thin film electroluminescent device of ZnS; Er3+ were studied. The results indicate that besides Er3+ substituting for Zn2+ as luminescent centers, the dominant point defects are sulfur vacancies, zinc vacancies, shallow donors and deep acceptors. Their influence on electroluminescence is discussed.

Investigations of diffusion of 35S radioisotope in β-Ni 3S 2 polycrystals

The diffusion of 35S radioisotope in β-Ni 3S 2 polycrystals was studied at temperatures ranging from 848 K to 893 K and at sulphur vapour pressure interval (5.6 × 10 −6−3.2 × 10 −4) Pa. The autoradigraphy and the tracer sectioning method were used. It was found, that the bulk diffusion is the main process of sulphur transport. The activation energy of the diffusion equals (190 ± 10) kJ mol −1. Under the conditions used in the experiments the dominant anionic point defects are single-ionized vacancies and quasi-free electrons.

Observation of singly ionized selenium vacancies in ZnSe grown by molecular beam epitaxy

Electron paramagnetic resonance (EPR) has been used to investigate singly ionized selenium vacancy VSe+ centers in ZnSe epilayers grown by molecular beam epitaxy (MBE). The study included undoped and nitrogen-doped films. Spectra taken at 8 K and 9.45 GHz, as the magnetic field was rotated in the plane from [100] to [010], showed an isotropic signal at g=2.0027±0.0004 with a linewidth of 5.8 G. In the two samples where this signal was observed, estimates of concentration were approximately 1.1×1017 and 6.3×1017 cm−3. The appearance of the EPR signal correlated with an increase in the Zn/Se beam equivalent pressure ratio (during growth) in undoped films and with an increase in the nitrogen concentration in doped films. We conclude that the singly ionized selenium vacancy may be a dominant point defect in many MBE-grown ZnSe layers and that these defects may play a role in the compensation mechanisms in heavily nitrogen-doped ZnSe thin films.

Generation of vacancy-type point defects in single collision cascades during swift-ion bombardment of silicon

Silicon samples of n-type have been implanted with low doses (${10}^{7}$-${10}^{10}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$) of $^{11}\mathrm{B}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{28}\mathrm{Si}$, $^{74,76}\mathrm{Ge}$, and $^{120}\mathrm{Sn}$ ions using energies between 0.4 and 8.0 MeV. Because of the low doses employed, single-collision cascades prevail, and an analysis of the implanted samples by deep-level transient spectroscopy (DLTS) reveals two dominant point defects of vacancy type, the divacancy (${\mathrm{V}}_{2}$) and the vacancy-oxygen (VO) center. The generation of ${\mathrm{V}}_{2}$ and VO is studied in detail as a function of ion mass, dose, dose rate, sample depth, and sample temperature. Surface-enhanced annihilation of migrating defects is found to play a significant role in depleting the concentration of ${\mathrm{V}}_{2}$ and VO centers in proximity to the surface. The concentration of ${\mathrm{V}}_{2}$ and VO increases linearly with ion dose, but the generation efficiency per incoming ion decreases at high dose rates after displaying a constant value below \ensuremath{\sim}${10}^{8}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. This dose rate effect exhibits a dependence on ion mass, and is qualitatively predicted by computer simulations of the defect reaction kinetics, applying a model where the interaction between individual collision cascades is primarily due to fast-diffusing Si self-interstitials. At dose rates \ensuremath{\leqslant}${10}^{8}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, the generation of ${\mathrm{V}}_{2}$ centers per ion-induced vacancy in the damage peak region is identical, within \ifmmode\pm\else\textpm\fi{}10%, for all the different types of ions studied. However, the ${\mathrm{V}}_{2}$ centers formed by heavy ions are strongly perturbed, as shown by large deviations from a one-to-one relation between the two DLTS peaks associated with the singly and doubly negative charge state of ${\mathrm{V}}_{2}$, and also by a broadening of the two peaks. In contrast to that for ${\mathrm{V}}_{2}$, the generation of VO centers per ion-induced vacancy decreases with increasing ion mass, consistent with the picture that light ions are more effective in generating point defects than heavy ions. At elevated implantation temperatures, gradual relaxation of lattice strain occurs together with recrystallization of disordered zones, promoting the formation of unperturbed ${\mathrm{V}}_{2}$ centers and increasing the generation efficiency of VO centers.

Point defects and the binding energies of boron near defect sites in Ni3Al: A first-principles investigation

First-principles local-density-functional calculations have been used to investigate the equilibrium point defect structure and boron-defect interactions in Ni3Al. The dominant point defect types in off-stoichiometric Ni3Al are substitutional antisite defects on both sublattices. The boron binding energy is dependent on lattice coordination; it is strongest near vacancy sites with a nearst-neighbour nickel coordination number of about four (instead of six as in the defect-free interstitial site) and with no aluminium atom nearest-neighbours. This suggests that boron tends to segregate to “open” defect sites and to enhance cohesion through the formation of localized Ni-B covalent bonds. Comparison of the binding energies of boron and carbon in Ni3Al shows that boron has a stronger tendency to segregate to “open” sites than carbon.

Oxygen Nonstoichiometry and Defects in Mn‐Doped Gd2Ti2O7+x

The oxygen nonstoichiometry in Mn‐doped Gd2Ti2O7, Gd2(Ti0.975Mn0.025)2O7+x, was measured electrochemically, as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell. The analysis of the data shows that the defect equilibrium can be described by considering the dominant point defects to be neutral oxygen interstitials, doubly charged oxygen vacancies, and trivalent and quadrivalent Mn ions substituted in the Ti sites. The enthalpies for the formation of neutral oxygen interstitials and trivalent Mn are determined.

Surface and bulk point defect generation in Czochralski and float zone type silicon wafers

The origin of the defects which mediate dopant diffusion in silicon has never been conclusively established. Two primary theories, one favoring bulk generation and the other surface generation, have been proposed to explain the source of point defects. However, the inability to experimentally isolate surface effects from bulk effects has presented experimental determination of the dominant point defect source. An experimental structure employing multiple box-shaped boron marker layers on a variety of silicon substrates has generated strong evidence for separate bulk and surface generation of point defects. A method of using dislocations generated by unstably strained SiGe layers as a sink of interstitials allows a determination of the relative magnitude of surface interstitial generation and bulk interstitial generation. Analysis reveals that the wafer surface under inert anneals is a significant source of point defects and that the interstitial source in the bulk is dependent on the oxygen concentration and precipitate size.

First Principles Computer Simulation of the Defect Chemistry of Rutile TiO2

AbstractThere exists a long-standing controversy concerning the nature of the dominant point defect mechanism in rutile TiO2. Previous classical shell model calculations by Catlow el al [1] find a strong preference for Schottky as opposed to Frenkel-type defects, lending support for oxygen vacancy rather than titanium interstitial compensation in reduced rutile. However, reviews of experimental studies [2], show that many conflicting conclusions have been reached. Ab initio total-energy calculations have been performed on a parallel computer to help resolve this controversy. First results indicate a Schottky formation energy (of the bound Schottky trio) consistent with the Mott-Littleton values of Catlow et al [1]. A first attempt is made at calculating the heat of reduction through determination of the formation energy of a neutral oxygen atom vacancy. As a. result some interesting insight is gained into the redox chemistry of reduced rutile.

Study of the influence of the nonequilibrium point defect concentration gradient on the dopant flux during ion implantation in silicon at high temperatures

A relationship in a closed form for a dopant flux in silicon in the presence of the nonequilibrium point-defect concentration and its gradient is derived from the first principles on the basis of solving a simplified boundary value problem. The experimental dopant atom distributions (profiles) obtained after ion implantation of B and P into Si at high temperatures are treated with the help of this relationship. The calculation results show that the exponential dependence of a dopant diffusivity at a distance from the surface can be attributed to the superposition of two effects: the presence of an exponentially decreasing excess of the point-defect concentration versus depth and the influence of the point-defect concentration gradient on the total dopant flux. Omission of the flux component attributed to the nonequilibrium point-defect concentration gradient can result in an overestimation of the point-defect density by more than one order. The conceptual model of the radiation-induced dopant redistribution formulated in terms of a generalised (complete) boundary value problem is reduced to a simplified one by using the assumptions: (i) the dominant interaction of dopant atoms with only one kind of the point defects; (ii) the quasi-equilibrium between the subsystems of the dominant point defects and the complex defects (dopant atom + point defect); (iii) the adiabaticapproximation for the nonequilibrium dilute solutions of dopant atoms and point defects in solids.

Defect chemistry of Cu 2− yO at elevated temperatures. Part II: Electrical conductivity, thermoelectric power and charged point defects

The electrical conductivity and the Seebeck coefficient of Cu 2O were measured as a function of temperature and oxygen partial pressure. The measurements were performed between 900 K and 1300 K and between 10 −12 atm and 0.15 atm. The results indicate that the dominant electronic charge carriers are holes, although at high temperatures (⩾ 1200 K) and low oxygen partial pressure (⩽ 10 −5 atm) there is also a significant contribution of electrons to the electrical conductivity and Seebeck coefficient. The dominant ionic point defects are doubly charged oxygen interstitials O″ i, dominating at temperatures above 1150 K, and singly charged copper vacancies V′ cu, dominating at temperatures below 950 K. The values of the enthalpy and entropy for the formation of the charged defects were found. The mobility values of holes and electrons were determined in the temperature range of 1000 K ⩽ T ⩽ 1250 K. The hole mobility is 3 ⩽, v h ⩽ 6 cm 2/Vs, and it decreases when the temperature increases. The electron mobility is higher than that of holes, with values of 150 ⩽ v e ⩽ 200 cm 2/Vs. The variation of the Fermi level within the Cu 2O phase as a function of oxygen partial pressure and temperature was also determined.

Compositional dependence of cation impurity gettering in Hg1−xCdxTe

Cation impurity gettering in Hg1−xCdxTe is described in the context of process models which include the interactions of the impurities and the dominant native point defects. Experimental results are presented using secondary ion mass spectroscopy (SIMS) profiles of Au redistribution in Hg1−xCdxTe (x = 0.2,0.3,0.4) following Hg anneals and ion mills, which are processes known to inject excess Hg interstitials. In either process, the IB impurity distributes preferentially to high vacancy regions. The junction depth of the low to high impurity transition is determined by SIMS. For Hg-rich anneals of Au-doped high vacancy concentration material, the impurity junction behavior with respect to anneal time and temperature is compared to that expected for type converted electrical junctions in vacancy-only material. For milled Au-doped Hg0.7Cd0.3Te with a high vacancy concentration, the impurity junction depths are approximately proportional to the amount of material removed, as was the case with x = 0.2 material. Hg anneal type-conversion rates are found to have a strong compositional dependence which compares favorably with the strong self-diffusion coefficient dependence on x-value. In contrast, the mill conversion rate has a weak x-value dependence. Effects of trace vs dominant Au levels compared to the background vacancy concentration are quantified. True decoration of intrinsic defect processes requires Au <<[Cation Vacancies].

Tracer-diffusion of manganese in α-MnS polycrystals

Self-diffusion of Mn in powder compacts of α-MnS has been studied with the use of the radioactive tracer 54Mn. The experiments were carried out between 1073 K and 1323 K and in the sulfur vapour pressure interval (10 ÷ 4 · 10 3) Pa by means of the tracer-sectioning method. It was confirmed that under the conditions used in this experiment the dominant point defects are doubly charged vacancies and electron holes. The activation energy of the diffusion equals (142 ± 10) kJ/mol.

Defect chemistry of Cu 2−yO at elevated temperatures. Part I: Non-stoichiometry, phase width and dominant point defects

The deviation from stoichiometry in Cu 2− y O was measured as a function of oxygen partial pressure, 10 −13≤ P O2≤0.1 atm, and temperature, 873≤ T≤1245 K, using a solid state electrochemical method. The measurements covered the whole width of the phase Cu 2− y O for the temperature range 873≤ T≤1245 K. The y− P O2− T relations are analyzed in order to identify the dominant point defects. It is found that in the high P O2 range of the phase, the dominant defects are neutral copper vacancies, V x Cu. In the low P O2 range there are two types of defects, dominating at different temperatures. At low temperatures the dominant defects are neutral copper interstitials, Cu x i. The analysis indicates that at temperatures higher than 1245 K the dominant defects are oxygen vacancies, V x O. The enthalpy and entropy for the formation of V Cu x and Cu i x are also determined.

Point defect diffusivites in San Carlos olivine derived from reequilibration of electrical conductivity following changes in oxygen fugacity

The time dependence of electrical conductivity following step changes in oxygen fugacity has been used to deduce the diffusivities of the dominant point defects in pyroxene‐buffered San Carlos olivine. Nonlinear least squares fits of the data indicate that two relaxation times (τ) are present in the conductivity‐time records. The effective diffusivity calculated from the slower τ agrees with values reported for the chemical diffusivity of metal vacancies (VMe‖). The effective diffusivity calculated from the faster τ coincides with that measured for analogous changes in strain rate (έ) following step changes in fO2. This faster diffusivity has tentatively been associated with the chemical diffusivity of silicon vacancies (VSi‖‖). These results imply that defects on the tetrahedral and octahedral sublattices of olivine may reequilibrate independently. Furthermore, changes in electrical conductivity and strain rate resulting from changes in thermodynamic conditions are initiated by reequilibration of the faster of these defects.

Investigation of the dominant point defects in tetragonal YBa2Cu3Ox at elevated temperatures

The manner in which oxygen is incorporated into YBa2Cu3O x (YBCO) at ∼800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O i x . The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O2), as a function of compositionx and temperatureT (5.99≤x≤ 6.35, 825≤T≤1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O i x are noninteracting forx ≃ 6. (b) The stoichiometric composition of YBCO isx ≃ 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O2. (d) The Fermi level changes by ≃ −0.2 eV asx increases from 6.05 to 6.35.

Native defects and diffusion in amorphous silicon -- a revisit

Recent theoretical and experimental results are assessed as they relate to the structure and properties of the dominant native paramagnetic point defect (D center) and the mechanisms of H diffusion. These results lead us to propose that the D center is neither a dangling or a floating bond, but an intermediate configuration which we label “frustrated bond”. Experimental results by Jackson et al. set an upper bound for the diffusivity of D centers, but are not incompatible with D centers mediating H motion. Doping- and illumination-enhanced H diffusion can be accounted for in a unifying manner in terms of excess D centers that are mobile and mediate H motion via the kick-out mechanism, without any new assumptions.

Nonideality of the solid solution in (U, Pu)O 2 nuclear fuels

The experimental investigation on properties of ( U 1− y Pu y ) O 2± x shows a deviation in the ideality for the solid solution. Our results on the unit cell parameter, the electrical conductivity, the specific heat, and, the thermal diffusivity have been analyzed to validate this conclusion. The variations in the properties have been interpreted from this new thermodynamical criterion, and, two transitions in the mechanism of U/Pu substitutions in the mixed oxide have been proposed. They occur for two particular plutonium concentrations: 3.1 and 12.5 at%. New types of clusters (Pu'-V o) or (Pu'-V'' o-U') are introduced to represent the dominant point defect in the structure of the hypostoichiometric mixed oxide for the MOX fuel compositions. The classical ionic model including the clusters (Pu'-V'' o-Pu') or (Pu'-V' o-Pu) can be dominant only above 15 at% of plutonium, i.e. for FBR fuels.

The association of compositional fluctuations with clusters of cavities in irradiated alloys

Abstract The microstructure of Fe-12% Cr-15% Ni alloys irradiated with neutrons at 645°C has been studied in detail. These materials display intragranular fluctuations in composition with an irregular size and spacing. The alloys also contain a distribution of small, helium-filled cavities. It is shown that these cavities are clustered, and that the clusters of cavities are associated with regions rich in nickel and depleted in iron and chromium. It is believed that the cavities are the dominant point defect sinks and that the nickel enrichment associated with the clusters of cavities is a result of radiation-induced segregation. Possible explanations for the clustering of cavities are discussed.

Modeling of Damage Enhanced Diffusion of Implanted Boron in Silicon

AbstractModeling of the damage enhanced diffusion (DED) behaviors of implanted boron in silicon of Powell’s experiment [1] has been performed. In his experiment, Powell showed that the diffusion of implanted boron in silicon was dependent on implantation dosage as well as on the annealing conditions. For low dose boron implantation, the extent of boron diffusion after 15 second RTP is less than that of furnace annealing at 900°C for 30 minutes. But the reverse is true for the high dose case, and a two-step annealing leads to least and minimal diffusion. In this work, implantation induced excess self-interstitials which generate mobile boron atoms at the intersititial sites are considered the dominant point defect species responsible for the DED. Both the local relaxation and diffusion of these excess self-interstitials are considered. The features of the DED reported by Powell are successfully reproduced and explained.

Native Point Defects and Nonstoichiometry in GaAs (I). Homogeneity Region of Gallium Arsenide

AbstractHomogeneity region and the concentration of dominant native point defects along its boundary are calculated by the method of quasichemical reactions with the use of experimental data on the structure type of solid solutions formed by the components in GaAs. The entropy and enthalpy of the formation of dominant native point defects in GaAs are obtained. Temperature ranges of the formation of microdefects‐finely dispersed Ga and As precipitates are considered.