Point Defect Populations Research Articles

The efficiency of displacement cascades for the generation and absorption of self-interstitials and vacancies, in a neutron irradiated AgZn alloy

Abstract Strain relaxation measurements were performed during irradiation with reactor neutrons to monitor the enhancement of the stress-induced directional ordering rate associated with the supersaturated point defect population, in a Ag-24 at. % Zn alloy. The corresponding rate evolution was followed from the first seconds after the initiation of flux until long irradiation periods, directly at temperatures where both self-interstitials and vacancies are mobile. The generation of freely migrating elementary defects by the displacement cascades was found to be characterized by a mean cross-section of 160 barns, if referred to fast neutrons (E>1 MeV) in the reactor spectrum, with a bias of a few per cent. From comparison with the estimated total defect production, it was deduced that only 10% of the initially created point defects undergo random diffusion. The production rate of extra sinks for these defects by cascade collapse was determined also, for irradiation temperatures between 0.3 and 0.4 melting temperature. In fact the measured cross-sections were strongly temperature dependent, as the net result of simultaneous sink formation and anneal during high temperature irradiation.

The Nature of Point Defects and their Influence on Diffusion Processes in Silicon at High Temperatures

ABSTRACTThe paper highlights recent progress in understanding the role of vacancies and self-interstitials in self- and impurity diffusion in silicon above about 700°C. How surface oxidation of silicon leads to a perturbation of the pointdefect population is described. An analysis of the resulting oxidationenhanced or -retarded diffusion of group III and group V dopants shows that under thermal equilibrium as well as under oxidation conditions both vacancies and self-interstitials are present. For sufficiently long times vacancies and self-interstitials attain dynamical equilibrium which involves their recombination and spontaneous thermal creation in the bulk of silicon crystals. The existence and the nature of a recombination barrier slowing down the recombination process are discussed in this context. Recent experimental and theoretical results on the diffusion of gold in silicon enable us to determine the selfinterstitial component of silicon self-diffusion and to obtain an estimate of the respective vacancy contribution. The two components turn out to be of the same order of magnitude from 700°C up to the melting point.

A dynamic intragranular fission gas behavior model

A model for the non-equilibrium behavior of intragranular fission gas in uranium oxide fuel is developed to study the fundamental phenomena that determine fission gas effects. The dynamic behavior of point defects and the variations in stoichiometry are explicitly represented in the model. The principle of distribution moment invariance is used to allow approximations that significantly reduce computational expense without sacrificing accuracy. A dynamic intragranular gas release and swelling (DIGRAS) computer code, that is based on the non-equilibrium model, was developed for both steady-state and transient applications. The code utilizes implicit multistep numerical integration methods, and is designed to give detailed information on all the physical processes that contribute to fission gas behavior. Simulations of steady-state irradiations indicate that the gas bubble re-solution process is very significant and results in very few large bubbles. The assumptions of equilibrium bubble sizes for normal steady-state irradiations in fast reactors appears to be adequate. On the contrary, a fully dynamic fission gas and point defect treatment was found necessary for transient simulations. The fuel stoichiometry was found to play an important role in determining bubble kinetics. This is mainly due to the strong dependence of point defect populations on stoichiometry. In fast transients, bubbles were found to be highly overpressurized, which suggests that a mechanistic plastic growth model is also needed.

Ion implantation damage in GaAs: A TEM study of the variation with ion species and stoichiometry

GaAs samples have been implanted with a dose of 2 × 10 14 cm −2 of each ion in the following combinations: Ga, As, Ga + As, Se, Ga + Se and As + Se. Implantation was at 200°C, and post implantation annealing at 700°C. Subsequent examination by transmission electron microscopy (TEM) showed clear and reproducible differences in the dislocation loop size and density, depending on the ion species implanted. The simplest results were obtained with the single implants, particularly Ga and As. These observed variations could be explained in terms of point defect populations, and hence rates of annealing at a given anneal temperature, being affected significantly by the stoichiometric effect of the implant. These simpler aspects were also seen to be incorporated in the more complex “dual” implants.

Correlation between dielectric constant and defect structure of non-stoichiometric solids

TO understand the properties and behaviour of inorganic non-stoichiometric compounds it is vital to know how the stoichiometric imbalance in the material is accommodated structurally. It has been usual to assume that point defect populations have been solely responsible for such departures from the normal stoichiometric formula. But, recent structural investigations have shown that for a growing number of materials this does not seem to be true and instead changes in the anion–cation ratio are accommodated by planar faults. This report points out that an empirical correlation seems to exist between the magnitude of the low frequency dielectric constant of a compound and the structural mode by which it accommodates its non-stoichiometry. Those materials with a low dielectric constant are found to prefer point defect populations, while those with a high dielectric constant prefer planar faults. Some reasons why this correlation should hold are suggested.

Deformation of single crystals of gallium arsenide

Deformation of single crystals of gallium arsenide is reported in bend and tension up to 1000° C whilst maintaining stoichiometry in an arsenic atmosphere. Surface defects and impurity segregation are shown to be dislocation sources. The dislocation density is low enough, however, to show large yield drops which are analysed in detail. Strains of 39% are possible. The activation energy for dislocation movement is increased by heat-treatment owing to an increase in point defect population.

Electrical and electron microscope studies of the annealing of tellurium-doped gallium arsenide

Abstract Hall effect measurements and electron microscope examination of tellurium-doped gallium arsenide crystals annealed at various temperatures indicate that tellurium and copper (present as a contaminant) produce a high population of matrix point defects which are probably simple vacancies. Annealing at temperatures below 1000°C causes the growth of intrinsic prismatic vacancy loops on {111} and {100} planes in association with decorations of precipitated copper. Heating to temperatures above 1000°C caused the copper and tellurium (in extrinsic stacking faults) to move back into solution, eventually changing the semiconductor from n to p type, and leading to the disappearance of both prismatic loops and stacking faults.