Condensation Of Defects Research Articles

Cross-sectional transmission electron microscopy parallel to the active stripe of degraded buried heterostructure distributed feedback laser devices

We demonstrate for the first time that cross-sectional transmission electron microscopy can be applied parallel to the active stripe of a distributed feedback buried heterostructure laser diode to identify the cause for degradation during reliability qualification. A dominant defect mechanism is found to be the generation of dislocation loops at or near the grating/waveguide-layer interface which subsequently propagate up through the multilayer structure. The vertical segments of the loops tend to be trapped onto the {111} planes to form a V-shaped configuration as revealed in the (011) cross section. A dislocation reaction mechanism is proposed to explain the observed dislocation configuration. Furthermore, complicated dislocation loops are grown out of the segments of dislocation threading through the active region forming <100≳ dark line defects revealed by the electroluminescence. The defect growth mechanism is believed to be a condensation of point defects induced by the nonradiative recombination assisted point defect migration process similar to that previously observed in degraded channeled substrate buried heterostructure lasers. The nucleation of dislocations at or near the grating/waveguide layer interface is consistent with a high interfacial strain observed in the degraded devices.

Topological phase transition in a two-dimensional nematic n-vector model: A numerical study.

A Monte Carlo study, based on a cluster algorithm, is made of the two-dimensional ${\mathrm{RP}}^{\mathit{n}\mathrm{\ensuremath{-}}1}$ model. Introducing various types of order parameters, we find strong evidence for a topological phase transition, driven by a condensation of defects. For n=3, the model describes a nematic liquid crystal. In this case, the transition is associated to a divergence of the correlation length and of the susceptibility, and a cusp in the specific heat. The results for n=40 are compared with the analytical ones for n=\ensuremath{\infty}. The critical temperature is correctly reproduced, but the detailed nature of the transition is not. Crossover phenomena are detected, which reflect the noncommutativity of the n=\ensuremath{\infty} and the thermodynamic limit for some observables.

Relationship between dislocation generation, vapour phase supersaturation and growth rate in InP layers obtained by vapour phase epitaxy

Abstract The interrelationship between growth conditions, dislocation generation mechanisms and layer growth rate has been studied in InP/InP layers prepared by hydride vapour phase epitaxy. In the present experiments, only the PH3 flow rate was varied, whereas the HCl flow rate was kept constant. Contrary to what is generally believed, even very low growth rates yield a very high dislocation density. The dislocations are generated by the condensation of point defects that are incorporated in supersaturation into the layer during the whole growth sequence. Under higher flow rate conditions, dislocation generation by glide processes seems to prevail. Layers with minimum defect density are obtained for intermediate PH3 flow rates. The influence of either the spiral growth or the two-dimensional nucleation mechanism on the macroscopic layer growth is discussed. A procedure to reduce the morphological defects, termed hillocks, is also suggested.

Numerical evidence for a topological phase transition in a two-dimensional spin model

A Monte Carlo study, based on a cluster algorithm, is made of the two-dimensional RP n−1 model. Introducing new types of order parameters, we find strong evidence for a topological phase transition driven by a condensation of defects. For n=3, the model describes a nematic liquid crystal.

First-order phase transition in the two- and three-dimensional RPn+1and CPn-1models, in the large-n limit

Solving the lattice RPn-1 and CPn-1 models in the large-n limit, the authors show that they undergo a first-order phase transition for dimensions greater than or equal to two. In the latter case, although there is no long-range order, they argue that at least in the RPn-1 model, the transition corresponds to a condensation of defects.

Topological defect condensation and high-temperature superconductivity.

The properties of bound states of holes and topological point defects of the s = 1/2 antiferromagnet are studied. Cooper pairs are formed by point defects of opposite topological charge but net 2e electronic charge. The superconductive transition leads into an insulating phase with respect to the topological charge.

Atomic modelling of homogeneous nucleation of dislocations from condensation of point defects in silicon

Abstract An energetically favourable atomic modelling scheme for homogeneous nucleation of dislocations in silicon by condensation of point defects is described. For extrinsic dislocation dipoles, a chain of interstitial atoms is used to form intermediate defect configurations having non-six-membered atomic rings with matrix atoms. For intrinsic dislocation dipoles, a chain of matrix atoms is cut out and the remaining atoms surrounding the cut are used to form intermediate defect configurations having non-six-membered atomic rings. Climb and glide motions of the intermediate defect configurations then produce the 90° edge, the 60° and the Frank partial dislocation dipoles. The intermediate defect configurations and the dislocation dipoles generated have {110} rod-like morphologies. A model with all four-coordinated interstitial atoms for {113} stacking fault is also obtained.

Mechanisms of radiation induced creep and growth

Irradiation creep occurs primarily because the applied stress causes the evolving microstructure to respond in an anisotropic fashion to the interstitial and vacancy fluxes. On the other hand, irradiation growth requires the response to be naturally anisotropic in the absence of applied stress. Four fundamental mechanisms of irradiation creep have been conjectured: stress induced preferred absorption (SIPA) of the point defects on the dislocations, stress induced preferred nucleation (SIPN) of point defects in planar aggregates (edge dislocation loops), stress induced climb and glide (SICG) of the dislocation network and stress induced gas driven interstitial deposition (SIGD). These mechanisms will be briefly outlined and commented upon. The contributions made by these mechanisms to the total strain are not, in general, mutually separable and also depend on the prevailing (and changing) microstructure during irradiation. The fundamental mechanism of irradiation growth will be discussed: it is believed to arise by the preferred condensation of point defects and climb of dislocation loops and network on certain crystallographic planes. The preferred absorption and nucleation is thus a consequence of natural crystallographic anisotropy and not due to any external stresses. Again the effectiveness of this mechanism depends on the prevailing microstructure in the material. In this connection attention will be particularly drawn to the significance of solute trapping, segregation at grain boundaries, dislocation bias for interstitials and transport parameters for an understanding of irradiation growth in materials like zirconium and its alloys; the relevance of recent simulation studies of growth in such materials using electrons to the growth under neutron irradiation will be discussed in detail and a consistent model of growth in these materials will be presented.

Swirl Defects in As-Grown Silicon Crystals

ABSTRACTDuring melt-growth of macroscopically dislocation free bulk silicon crystals (floating-zone and Czochralski technique) microdefects can form due to the condensation of thermal point defects (self-interstitials, vacancies). The formation of these imperfections, generally referred to as “swirl defects”, is strongly affected by the growth conditions (e.g. the crystal pulling rate) and crystal purity. The various reported defect formation models will be discussed. Special attention will be paid to the effect of doping on swirl defect formation.

The behaviour of materials in fast reactors

Abstract Fast neutron damage in fast reactors can limit the life of structural components through the growth of voids. The main features of the current theory of point defect production and condensation are surveyed. The role of metallurgical structures and radiation produced extended defects is outlined and used to demonstrate the development of volume swelling and radiation hardening. Mechanisms of radiation creep are described in the context of the preceding treatment of point defect behaviour. Finally, future trends in the field are briefly explored.

Microdefects and striations in dislocation-free LEC-GaP crystals

Dislocation-free GaP crystals grown by the liquid encapsulated Czochralski technique were characterized chiefly by the use of X-ray transmission topography and EBIC-mode scanning electron microscopy. Preliminary results obtained with transmission electron microscopy are also presented. X-ray topographs reveal a crystal inhomogeneity which is mainly attributed to the presence of non-stoichiometric striations. High densities (10 11-10 12 cm -3 of microdefects were detected, distributed both homogeneously and in a striated pattern. Various types of microdefects (perfect and faulted dislocation loops and (semi) coherent precipitates) were revealed. Although most microdefects are less than 1 μm across, significantly larger defects can form. Evidence for macroscopic dislocation generation due to point defect condensation is presented.

The introduction of dislocations during the growth of floating-zone silicon crystals as a result of point defect condensation

During growth of dislocation-free floating-zone crystals two types of “swirl defects” (A- and B-clusters) are formed as a result of point defect condensation. The diffusion coefficient of these point defects (vacancies, silicon interstitials) in the temperature interval 1050–1100 °C, is found to be 2 × 10 -5 cm 2/sec. Section topographic analysis showed that the A-clusters (dislocation loops) rapidly increase in size with decreasing crystal cooling rate. This is attributed to a combined climb-glide process. Evidence is presented that the predominant cause for the loss of dislocation-free growth, particularly of large-diameter crystals, is due to emission of dislocation arrays from the A-clusters.

Neutron irradiation damage in SiC whiskers

Abstract Radiation damage in SiC whiskers has been observed using transmission electron microscopy. Damage is apparent as spots up to 5 nm diameter at neutron doses in excess of 2 × 1020 n.v.t.  0·18 MeV. At the highest dose employed (∼1·2 × 1021 n.v.t.0·18 MeV), agglomeration of defects appears to have taken place. Further evidence for defect condensation was obtained on examination of vacuum-annealed irradiated whiskers, when dislocation loops lying on {111} were observed.

The loss of coherency of precipitates and the generation of dislocations

Abstract The theory of Brown, Woolhouse and Valdre (1968) is extended to include the possibility of the nucleation of interface dislocations by prismatic punching and the condensation of point defects. It is shown that neither process can occur spontaneously (i.e. without the introduction of dislocations by another process such as deformation or irradiation) unless the misfit exceeds a critical value, estimated to be 0·05. Furthermore, unless the radius of the precipitate exceeds certain critical values, the processes will not occur because the energy of the system is not thereby lowered. A comparison of the theory with experiment enables limits to be placed on the stress necessary to cause the generation of dislocations in a perfect lattice, and at an incoherent interface. In the former case the generation stress is higher than hitherto supposed; but in the latter case it appears that an incoherent interface behaves as an ideal source of dislocations, in the sense that when the energy of the system can b...

Length and electrical resistivity changes of neutron irradiated uranium

Abstract The length change of uranium single crystals in the [100], [010], and [001] crystallographic directions was determined during thermal neutron irradiation at temperatures between 5°K and 550°K. The electrical resistivity induced in uranium single crystals during neutron irradiation at temperatures between 4·5K and 20°K was also determined. The recovery of the irradiation-induced electrical resistivity and length changes was correlated with the temperature dependence of the irradiation-induced length changes. These results are consistent with the hypothesis that elongation in the [010] direction and contraction in the [100] direction during thermal neutron irradiation of uranium single crystals are due to the anisotropic condensation of vacancy and interstitial defects in planar clusters. The substantial decrease in magnitude of the length changes on increasing the irradiation temperature from 20°K to 140°K was attributed to the annihilation of interstitials at vacancy defects because of interstiti...

Dislocation loops in irradiated iron

Abstract Dislocation loops have been observed in iron foils irradiated with protons or Fe+ ions of energy 150 kev. Those produced by irradiation with Fe+ ions are shown to lie on {100} planes and to be pure edge in character. They are normally square, with the sides in the corresponding directions, and are formed by the condensation of interstitial atoms. It is believed that the loops are formed because of the excess concentration of interstitial atoms resulting from the direct insertion of atoms on bombardment. It is shown that either a homogeneous or a heterogeneous nucleation mechanism can account for the loop density observed. Finally, an estimate is made of the strain energy of various modes of condensation of point defects in iron. The most favoured are ½[111] (110), ½[111] (111), and [100] (100).

Production of Dislocation Loops by a Combined Climb and Glide Mechanism

Studies of dislocations in elongated aluminum crystals by diffraction electron microscopy revealed narrow dislocation loops lying parallel to 〈112〉. These loops formed only behind screw dislocations, their long parts then having edge character. Four mechanisms have been proposed to account for the formation and stabilization of the narrow loops through the condensation of point defects.

Stacking Faults in Zinc

Configurations of individual dislocations in zinc have been studied by electron transmission microscopy and by x-ray diffraction microscopy. Loops of sessile dislocation have been observed surrounding stacking faults lying on (0001) planes. These loops were evidently formed by condensation of point defects. Changes in loop size with climb of the surrounding dislocation have been observed occurring in the electron microscope. Separation of glissile dislocations into partial dislocations separated by stacking faults on (0001) planes have also been observed. Various interactions among the several types of dislocations occur.