Source Of Point Defects Research Articles

Origin of Au-Ag Mineralization in Sphalerite Ores from Data on Sphalerite Co-Crystallization with Ag and Au in Model Hydrothermal Systems

Abstract ––Au-Ag mineralization occurrences in sphalerite ores of hydrothermal genesis are paradoxical in view of the incompatibility of these elements in sphalerite. The formation of sphalerite with Au and Ag impurities under hydrothermal crystallization of ZnS at 450 °C and 1 kbar pressure was studied experimentally. Sn impurity was taken as a source of point defects in crystals modelling the interaction of Au and Ag with vacancies. The Ag solubility in low-Fe sphalerite is estimated as 3.8 ± 0.7 μg/g, Au ̶ ≤ 0.6 μg/g. The main forms of Ag and Au occurrence in sphalerite are the inclusions of (Ag, Au)xS phases with x varies mainly from 1.8 to 2.0, and Au varies from 0.01 to 0.75 a.p.f.u. The primary forms of the elements in ores might be microinclusions (Ag, Au) 1.8-2.1S or close to (Ag, Au)S at higher fS2. In presence of Sn, solubilities of Au and Ag become higher. The behavior of Au corresponds to the substitution reaction Sn4+ + Au+ + v‒ ↔ 2Zn2+ in the presence of two types of vacancy defects (v–) ‒ the “inherent” vacancies dependent on the crystallization conditions and the vacancies accompanying Sn4+ incorporation. Ag entrance is seemingly more dependent on fS2 conditions and does not correlate with Sn. The extra vacancies arise because of metastable crystallization under the conditions of oversaturation of growth medium. This is supported by the spherulite morphology of growth products and the admixture of wurtzite ZnS form. The distribution and cocrystallization coefficients show an increasing trend for both precious metals (PM), due to which Au changes from incompatible to the category of highly compatible elements in sphalerite. The geochemical environments favorable for the formation of imperfect mineral crystals are considered. Such crystals are capable to uptake PMs and other incompatible in “ideal” crystal elements because of their interaction with vacancies, both constitutional (inherent to the substance) and non-equilibrium defects, and surficial nano-sized formations (nonautonomous phases). The evolution of these initially “invisible” forms of PM under metamorphic processes and remobilization of ore substance may result in Au and Ag escape and aggregation into microparticles.

The damage and healing of quartz under radiation at high temperatures

The effect of radiation on silica has attracted significant attention due to its potential application where radiation exists. However, the nature of the atomistic defects of quartz formed during the radiation-induced damage at high temperatures has not been fully elucidated. Molecular dynamics is adopted to investigate the damage-healing behavior of the quartz at high temperatures in this research. The ensuing diffusion and recovery of point defects in the irradiated specimen at high temperatures are simulated. The high temperature reduces the number of point defects. At 1400 K, all point defects are removed, while only some point defects are erased at 700 K. It is illustrated that the over-coordinated atoms are the main source of point defects at 300 K and 700 K. The temperature influences the configuration of point defects. The healing variable of the molecular dynamics model is subsequently developed to describe the healing process. The healing mechanisms of the irradiated specimen at high temperatures are summarized by analyzing the trajectories of atoms in detail. These results are helpful to understand the nature of radiation damage in quartz at high temperatures.

Role of Grain Boundaries under Long-Time Radiation.

Materials containing a high proportion of grain boundaries offer significant potential for the development of radiation-resistant structural materials. However, a proper understanding of the connection between the radiation-induced microstructural behavior of a grain boundary and its impact at long natural time scales is still missing. In this Letter, point defect absorption at interfaces is summarized by a jump Robin-type condition at a coarse-grained level, wherein the role of interface microstructure is effectively taken into account. Then a concise formula linking the sink strength of a polycrystalline aggregate with its grain size is introduced and is well compared with experimental observation. Based on the derived model, a coarse-grained formulation incorporating the coupled evolution of grain boundaries and point defects is proposed, so as to underpin the study of long-time morphological evolution of grains induced by irradiation. Our simulation results suggest that the presence of point defect sources within a grain further accelerates its shrinking process, and radiation tends to trigger the extension of twin boundary sections.

A new constitutive approach to large strain plastic deformation

Abstract This paper presents the results of studies regarding the effect of nonconventional plastic deformation on the mechanism of plastic flow of metals. The applied method, called KOBO extrusion, in which monotonic large deformation is accompanied by cyclic reversible plastic twisting, falls into the category of severe plastic deformation. The cyclic reversible plastic twisting, which proved to be redundant in terms of changes in the material's geometry, appears to be a particularly efficient source of point defects. The experimentally proven Newtonian viscosity of metals at low temperatures under KOBO extrusion conditions implies that it is necessary to conduct a study on the crystal plasticity from the point of view of the constitutive law in which a crystal's viscosity totally controls the kinetics of the process. The well known dependency of the viscosity parameter on the diffusion coefficient, which, in turn, is a function of point defect concentration, dictates the link between the frequency of reversible plastic twist and the crystal's viscosity and, consequently, determines the mode of plastic flow. The results of this work provide experimental verification which justifies this new approach to crystal plasticity.

Impurity incorporation in orientation patterned GaAs grown by low pressure HVPE

Orientation-patterned GaAs (OP-GaAs) has shown promise as an efficient frequency-shifted laser source over the range of 2–12μm. In order to make OP-GaAs a viable source, efficiency and output power must be significantly increased, which requires minimizing major sources of loss. Low pressure HVPE has been adopted as the most suitable technique for regrowth of thick high quality GaAs layers on OP templates. We have explored process parameters in bulk and OP material to identify and control the sources of point defects, a key contributor to optical losses. Growth on OP templates with periodic [001] and [00−1] domains results in domain specific surface orientation, which should have inhomogeneous defect incorporation. Hall measurements, SIMS depth profiling, and cathodoluminescence (CL) were used to identify point defects in bulk and OP-GaAs. It was found that Si impurities are the primary source of donors, while VGa were identified as the primary source of acceptors. In order to study the incorporation of impurities in OP-GaAs samples, we intentionally doped samples with Si to increase CL and SIMS detectability. Spatially resolved CL and SIMS revealed regions with significant differences in the defect concentration, which can affect device output.

Synthesis, characterization and electrical properties of CdI 2 doped Al 2O 3 and TiO 2 superionic conductors

Superionic conductors were prepared by admixing metallic oxides (alumina and titania) with cadmium iodide in different proportions using direct mixing method. The synthesized materials were characterized by various qualitative techniques such as XRD (X-ray diffraction), DSC (differential scanning calorimetry) and SEM (scanning electron microscopy). Pure CdI 2 shows the standard pattern which corresponds to β-phase stable at room temperature, however, the diffractograms of the mixtures shows two phase nature of the materials with no effect of the second phase on the peak positions of the first, i.e. CdI 2. DSC curves also confirmed the formation of composite. SEM micrographs show the presence of great number of space charge regions which are very important in creating a great number of surfaces which in turn act as additional sources of point defects. It was also found that the conductivity of CdI 2 increases with mole fractions of dopants till x = 0.5 for alumina and x = 0.3 for titania. Arrhenius equation was used to study the temperature dependence of electrical conductivity and the activation energy of pure cadmium iodide was found to be 0.792 eV.

Formation ofY2O3nanoclusters in nanostructured ferritic alloys during isothermal and anisothermal heat treatment: A kinetic Monte Carlo study

Kinetic Monte Carlo simulations, based on parameters obtained with density-functional theory in the local-density approximation and experimental data, are used to study bulk precipitation of ${\text{Y}}_{2}{\text{O}}_{3}$ in $\ensuremath{\alpha}$ iron. The simulation involves realistic diffusion mechanisms, with a rapid diffusion of O atoms by interstitial jumps and a slower diffusion of Fe and Y atoms by vacancy jumps, and a point defect source which drives the vacancy concentration toward its equilibrium value, during isothermal and anisothermal heat treatments. Depending on alloy and thermal history conditions, the Monte Carlo simulations predict different kinetic behavior, including transient precipitation of metastable iron oxides followed by precipitation of ${\text{Y}}_{2}{\text{O}}_{3}$ nanoclusters.

Kinetics of heterogeneous grain boundary precipitation of NbC in α-iron: A Monte Carlo study

We propose Monte Carlo simulations of the NbC grain boundary (GB) precipitation kinetics in α-iron, based on an atomic description of the main mechanisms that control the kinetic pathway. The simulation involves realistic diffusion mechanisms, with a rapid diffusion of C atoms by interstitial jumps and a slower diffusion of Fe and Nb atoms by vacancy jumps; a simple model of the GB which reproduces the equilibrium segregation properties of Nb and C; and a point defect source which drives the vacancy concentration towards its equilibrium value. Depending on the precipitation conditions, Monte Carlo simulations predict different kinetic behaviours, including early segregation of C atoms at the grain boundaries, transient precipitation of metastable carbides, and homogeneous and heterogeneous NbC precipitation.

Recrystallization of atomically balanced amorphous pockets in Si: A source of point defects

We use classical molecular dynamics simulation techniques to study the regrowth behavior of amorphous pockets in Si. We demonstrate that crystallization depends on the morphology of the pocket-crystal interface. Although our simulated amorphous pockets had not any excess nor deficit of atoms with respect to perfect crystal, after regrowth we found residual defects. Most of them are single Si interstitials and vacancies, but also larger defects have been encountered. We have determined their atomic structures and calculated their formation energies. These complexes are more stable than amorphous pockets, and may trigger the formation of extended defects or favor damage accumulation.

Atomistic Monte Carlo Simulations of Homogeneous and Heterogeneous Precipitation on Grain Boundaries of NbC in Steels

The precipitation of niobium carbides in industrial steels is commonly used to control the recrystallization process or the amount of interstitial atoms in solid solution. It is then important to understand the precipitation kinetics and especially the competition between homogeneous and heterogeneous precipitation, since both of them have been observed experimentally, depending on the alloy composition, microstructure and thermal treatments. We propose Monte Carlo simulations of NbC precipitation in α-iron, based on a simple atomic description of the main parameters which control the kinetic pathway : - realistic diffusion properties, with a rapid diffusion of C atoms by interstitial jumps and a slower diffusion of Fe and Nb atoms by vacancy jumps - a simple model of grain boundaries which reproduces the equilibrium segregation properties of Nb and C - a point defect source which drives the vacancy concentration towards its equilibrium value. Depending on the precipitation conditions, MC simulations predict different kinetic behaviours, including homogeneous and heterogeneous NbC precipitation, early segregation of C atoms and its importance as a first stage for NbC precipitation, wetting phenomena on grain boundaries and transient precipitation of metastable carbides.

Evolution of a Microstructure of Materials under Irradiation

The evolution of a microstructure of metals (or alloys) under irradiation resulting in swelling of the material is considered for the case with the formation of Frenkel pairs. A closed system of equations describing the evolution of the microstructure of the material exposed to irradiation is obtained, and relationships for the swelling rate are derived. It is shown that the swelling rate varies linearly with time for a stationary source of point defects (the number of Frenkel pairs per lattice site). An expression for the swelling rate is deduced for a radiation source operating in a more realistic pulsed mode.

Localized electron irradiation methods and their application to detection of flow-field of point defects.

We propose new techniques of electron irradiation for studying the kinetics of simple point defects and their agglomeration, by making the most use of the intrinsic function of an ultrahigh-voltage scanning transmission electron microscope. The methods enable us to realize a very high defect production rate at a localized area and create a spot or band-shape source of point defects. The present methods were applied to an attempt to detect the point defects flowing out of a very localized area, examining the growth and shrinkage of pre-introduced interstitial clusters. At low temperatures where vacancies are immobile, outflow of the interstitials was observed. In contrast, at an elevated temperature where vacancies are mobile, outflow of the vacancies seemed to play a dominant role in the apparent secondary defect reactions. A possible explanation to aid understanding of the present results is proposed.

Influence of Growth Parameters on the Deep Level Spectrum in MBE-Grown n-GaN

ABSTRACTThe impact of growth temperature and Ga/N flux ratio on deep levels in GaN grown by molecular beam epitaxy (MBE) is systematically investigated using both deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) in a study designed to map out the presence and concentration of defects over a defined region of the MBE GaN growth phase diagram. A series of Si-doped GaN films were grown to cover a substrate temperature range and a Ga/N flux ratio range that spans from the N stable to the Ga droplet regimes along both variables. Identical growth templates were used to eliminate variations in dislocations between samples so that point defect variations could be tracked. For these samples, traps are detected at EC-Et=0.25, 0.60, 0.90, 1.35, 2.40, 3.04, and 3.28 eV. The near valence bands states at EC–3.04 and EC–3.28 eV are found to be strongly dependent on Ga/N flux with decreased concentrations as a function of increasing Ga flux toward the Ga droplet regime, but with little effect from growth temperature. The EC-1.35 eV level shows a strong dependence on growth temperature and only slight dependence on Ga/N flux ratio. In contrast, the concentration of the EC-Et=0.25, 0.90 eV levels increased with increasing Ga flux toward the Ga droplet regime, while the EC-Et=0.60 shows no dependence. The variation in concentration of the EC-2.40 eV level that has been related to VGa was difficult to quantify, but tends to increase towards nitrogen rich growth. The dependencies for the detected states with respect to growth temperature and Ga/N flux ratio suggest different physical point defect sources.

Smoothing of the macrorelief on the surface of a solid of arbitrary stoichiometric composition under irradiation

A general system of equations is formulated describing the evolution of the macrorelief on the surface of a solid of arbitrary stoichiometric composition under irradiation. Irradiation is treated only as an additional source of point defects (vacancies and interstitial atoms). A general formula for the smoothing of the macrorelief with time is obtained with consideration of the real conditions of the problem. A two-component substance is considered as a special case: new forms of the dependence of the surface smoothing rate on the roughness parameters appear, which are not observed for a one-component material. The surface and bulk diffusion coefficients of the material, as well as the constants characterizing the interaction of radiation with the substance, can be found by experimentally investigating the surface smoothing rate as a function of the roughness.

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.

Electrical deactivation of arsenic as a source of point defects

The effect of electrical deactivation of arsenic in silicon has been studied. High concentrations of arsenic were implanted and laser melt annealed, creating boxlike fully electrically active arsenic layers, with no residual implant damage. Wafers were then subjected to low temperature thermal cycles while a buried boron layer monitored point defects. Strong enhancements in the boron diffusion were observed suggesting that arsenic deactivation releases large numbers of interstitials. This is explained by a process where the vacancies required by the deactivated arsenic structures are created through a deactivation assisted Frenkel pair generation process, thus injecting interstitials.

A Model to Explain the Variations of “End-of-Range” Defect Densities with Ion Implantation Parameters

ABSTRACTComputer simulations in conjunction with TEM experiments have been used to test the different models usually adopted in the literature to explain the formation of “End Of Range”(EOR) defects which appear after annealing of preamorphized silicon layers. Only one survives careful experimental investigations involving Si+, Ge+, Sn+ amorphization at RT and LNT. The “excess-interstitial” model appears relevant at least for a semi-quantitative explanation of the source of point-defects which after recombination and agglomeration, lead to the formation of these defects. This model may be used for the numerical optimization of conditions for the production of high performances ullra-shallow junctions.

Modeling Rapid Thermal Diffusion of Arsenic and Boron in Silicon

Rapid thermal annealing (RTA) of high dose B and As implants is believed to produce large transients of point defects that may exist for the duration of the anneal (2–30s). Enhanced diffusion during RTA of boron implants >1015 cm−2is related to damage clusters at the peak of the range which can grow into cross‐grid dislocation networks. This process can create point defects through dislocation‐dislocation reactions and by nonconservative climb processes. Enhanced diffusion during RTA of high dose As implants is related to the amorphous Si surface layer formed during implantation. Regrowth of this layer into a defect‐free, crystalline layer is assumed to produce point defects which remain behind until they diffuse away. In the meantime, they contribute to enhanced As diffusion during the first 5–10s of the RTA cycle. Preamorphorization at low temperature eliminates this enhanced diffusion of As by eliminating the source of point defects presumed to be vacancies because of the associated defect removal in the implanted layer. Enhanced dopant diffusion has been modeled by overlaying an empirical transient point defect model on thermally assisted diffusion models associated with steady‐state furnace environments. A weighted exponential function was used to describe the transient process with a decay time of 4.4s.

The rate equations used in irradiation studies

The main purpose of this paper is the investigation of the rate equations used in the theories of void nucleation and swelling as well as creep, in situations where random displacement cascades are the major source of point defects. The point-defect concentrations are described by diffusion equations with random source terms to account for the cascades. Using methods from nuclear reactor kinetic theory and averaging over time, the rate equations are derived. It is seen that the rate equations are valid for computing time and spatially averaged point-defect concentrations, regardless of the cascade statistics, if the observation time is greater than the mean time between cascade events in the irradiated material. An alternate approach is suggested for studying pulsed systems with short pulse widths.

POINT DEFECTS AND DISLOCATION CLIMB IN III-V COMPOUNDS SEMICONDUCTORS

This paper reviews the low temperature dislocation climb process in III-V compounds semiconductors and points out areas in which more experimental information is needed to understand this complex problem. A dislocation climb model requiring the supersaturation of point defects of only one element of the coumpound is found to account for the main climb features. Rapid dislocation climb is attributed to recombination enhanced defect motion. Finally evidence of an interaction between the climb dislocations and a deep level donor center suggest that it might possibly be associated with the source of point defects needed for dislocation climb in Ga1-xAlxAs structures.