Dislocation Images Research Articles

Simulation of threading dislocation images in X-ray topographs of silicon carbide homo-epilayers

Three types of dislocation are seen in homo-epilayers of SiC grown on 4H-SiC wafers with an 8° surface offcut: axial screw dislocations, basal plane dislocations propagated into the epilayer at an 8° inclination and threading edge dislocations. These types may be imaged by monochromatic synchrotron X-ray topography in the grazing-incidence reflection geometry using the 11\overline 28 reflection. Equations needed to apply the ray-tracing method of computer simulating X-ray topographic defect images in this experimental geometry were derived and used to simulate images of all three. Simulations for axial screw dislocations appear as white circles surrounded by narrow dark rings, and those for basal plane dislocations as linear white streaks, both consistent with experimental topographs. Simulations of the threading edge dislocations showed 4 µm wide white ovals with narrow arcs of dark contrast at their ends, inclined relative to the g vector of the topograph according to the sign of their Burgers vector. These images resembled the experimental topographs inasmuch as was possible at the maximum resolution of X-ray topographs.

The asymmetry of the lobe contrasts in two‐beam TEM as a test to obtain the Burgers vector of an inclined relaxing dislocation

AbstractIt is suggested that, if the elastic relaxation effect of an inclined dislocation at both free surfaces of a thin plate is taken into account, the determination of its Burgers vector b in two‐beam transmission electron microscopy can be obtained with a notable reduction of experimental work. The relaxation effect produces in general an asymmetric aspect of the lobe contrasts, with respect to a medium line perpendicular to the dislocation image segment. Indeed, from a re‐examination of about twenty micrographs of dislocations located in two different materials, it is concluded that a single experimental image is often sufficient to derive b. In no case three experimental images obtained with non‐coplanar diffracting vectors were found necessary to obtain unambiguously b.

A preliminary study of CdS for solar cells using combined TEM and cathodoluminescence

A study of the suitability of a combined transmission electron microscope (TEM)/cathodoluminescence (CL) imaging and spectroscopy apparatus for investigations of CdS is presented. Photoluminescence (PL) was used to evaluate the effect of the Ar + and I + ion beam thinning used in TEM specimen preparation of CdS: a minor increase in yellow emission (594 nm) resulted. However, excitation of luminescence spectra in the TEM had a quenching effect on red luminescence (734 nm), this being considered due to the high excitation density compared to that in PL. Significant electron beam damage to the CdS could be avoided by using scanning transmission electron microscope (STEM) illumination in preference to the conventional TEM mode, which generally uses higher beam current density. Dislocation images were correlated with contrast in the STEM-CL imaging mode. The potential of the apparatus to make further direct correlations of CL images with diffraction contrast TEM imaging was assessed using the Rose visibility criterion.

The thermal stability of and nature of damage in Ti alloys subject to high-strain-rate deformation

Samples of Ti–6 wt% Al–4 wt% V and Timet 550 (Ti–4 wt% Al–4 wt% Mo–2 wt% Sn–0.5 wt% Si wt%), which have been deformed at a strain rate of 5 s−1, were annealed after thinning so that the visibility of dislocations in transmission electron microscopy could be compared before and after annealing. It has been found that imaging with g = 0002 produces clear images of dislocations before and after annealing, but that imaging with other diffracting vectors gives reasonable dislocation images only after annealing to at least 700°C. The sharpness of Kikuchi lines in diffraction patterns obtained from fully annealed samples, deformed at 10−1 and 5 s−1, has been examined. The lines are sharp for all planes in the fully annealed samples, but become more diffuse in samples deformed at 10−1 s−1. However, in samples deformed at 5 s−1 the Kikuchi lines from (0002) planes are sharp but the lines from all other planes are diffuse. These observations are interpreted in terms of the presence of a high density of defects, which do not distort the elastically strong (0002) planes as significantly as they distort all other planes. These observations are discussed with respect to the recent claim that a particular Ti alloy deforms by a mechanism that does not involve dislocations.

HVEM characterization of crack tip dislocations in silicon crystals

Crack tip dislocations in silicon crystals have been studied using high voltage electron microscopy (HVEM). The dislocation images have been investigated by comparing the observed images to those that were simulated. Dislocation images were computed in the conditions g x b = 0, 1 or 2 (g: diffraction vector, b: Burgers vector). The characteristics of 60 degrees dislocations were examined in detail since these dislocations are not characterized only by the condition g x b = 0 criterion. The computed images were in good agreement with those observed by HVEM. Residual contrasts of 60 degrees dislocations under the condition g x b = 0 exhibited characteristic black-and-white contrasts, which can contribute to the characterization of the dislocations. Such characterization of dislocations is essential in order to clarify the dislocation mechanism which controls the fracture toughness of materials.

Determination of the three-dimensional structure of dislocations in silicon by synchrotron white X-ray topography combined with a topo-tomographic technique.

The determination of the three-dimensional dislocation structure, i.e. the configuration and nature of the dislocations, in silicon by synchrotron white X-ray topography combined with a topo-tomographic technique is demonstrated. A [001]-oriented CZ-Si crystal of diameter approximately 7 mm was fixed on a subsidiary goniometer with three rotation axes (omega, Rx, Ry) and x-y-z stages, keeping its growth axis parallel to the omega-axis. This goniometer was mounted on the swivel stage of the main diffractometer installed at the experimental hutch of beamline BL28B2 of SPring-8. After adjusting the (110) plane to be perpendicular to the incident white X-ray beam by observing transmission Laue patterns, the crystal was inclined by 4.3 degrees so that the 004 Laue spot could be formed by 60 keV X-rays. Laue topographs observed by rotating the omega-axis of the subsidiary goniometer were recorded using X-ray films and a cooled CCD camera. The direction of the dislocation lines, Burgers vectors and glide planes were determined by following the variation in features of the dislocation images in the 004 Laue spot and by examining the contrast of the dislocation images in the Laue spots concerning the [111] and [022] planes. It is concluded that white X-ray topography combined with the omega-rotation technique is useful for clarifying the three-dimensional dislocation structure.

A distributed-dislocation method for treating free-surface image stresses in three-dimensional dislocation dynamics simulations

This work reports on a recent advancement in three-dimensional dislocation dynamics modelling. A method is presented for the treatment of dislocation image stresses resulting from the presence of nearby traction-free surfaces. The image stress-field of a dislocation segment below a finite-sized free surface is obtained by the distribution of N generally prismatic rectangular or square dislocation loops padding the area (the external bounding surfaces of the simulation box). The unphysical tractions created at surface collocation points by sub-surface crystal dislocations are annulled by proper determination of the loops' Burgers vectors. The image stresses on a dislocation segment are simply those stresses resulting from the surface loops. The accuracy can be improved by increasing the number of collocation points (i.e. surface loop density).

The influence of strain rate on the visibility of dislocations in transmission electron microscopy images of deformed Ti–6 wt% Al–4 wt% V and in Timet 550

Samples of Ti–6 wt% Al–4 wt% V and Timet 550 (Ti–4 wt% Al–4 wt% Mo–2 wt% Sn–0.5 wt% Si) have been subjected to strain rates between 10−1 and 103 s−1and detailed examination of the dislocation structure in the α grains has been carried out using transmission electron microscopy (TEM). For samples deformed to a strain of 0.1 at 10−1 s−1, detailed analysis of the defects can be carried out using all diffracting vectors and the presence of (c + a) dislocations and a dislocations thus confirmed. In contrast, for samples strained to the same strain of 0.1 but at 5 s−1, it is not possible to obtain images of dislocations when using any diffracting vectors other than 0002. Thus the presence of dislocations which have a Burgers vector containing a c component can be confirmed in the samples strained at 5 s−1 but the presence of a-component dislocations can only be inferred from TEM of these samples because of the difficulty of obtaining images with diffracting vectors other than 0002. Limited observations on samples strained at 103 s−1 show that similar difficulties are found in imaging dislocations as are found in samples deformed at 5 s−1 but at this strain rate, the highest used, the difficulties are reduced since images can be obtained in some grains using diffracting vectors other than 0002. These results are discussed in terms of the nature of damage as a function of strain rate and the factors that influence contrast from dislocations in crystals.

Free Surface Relaxation and Two-Beam TEM Imaging of Straight Dislocations in Thin Foils

Free Surface Relaxation and Two-Beam TEM Imaging of Straight Dislocations in Thin Foils

Procedural sedation and analgesia for paediatric patients in the emergency department

Children presenting to the emergency department (ED) often require sedation for brief procedures such as fracture and dislocation reductions, laceration repairs, and imaging procedures that are painful, anxiety provoking or both. This article presents three cases of paediatric patients who require sedation and/or analgesia, and summarizes important aspects of procedural sedation for the primary care practitioner in the emergency setting. Presedation assessment and monitoring equipment are detailed. Discussion of routes of administration and different agents including barbiturates, opiates, benzodiaxepines, the 'cardiac coctail', ketamine, propofol, nitrous oxide, and etomidate follow. Emphasis is placed on indications, contraindications, dosing, timing and advantages and disadvantages of each. Reversal agents are mentioned, and discharge criteria are outlined.

Dislocation Image Stresses at Free Surfaces by the Finite Element Method

ABSTRACTThe finite element method has been routinely used to calculate the image stresses of dislocation segments. When these segments intersect with surfaces, the image stresses at the surfaces diverge singularly. At the presence of these singularities, both convergence and accuracy of using the finite element method need to be examined critically. This article addresses these issues with the aim toward the application of dislocation dynamics simulations in thin films.

Surface-relaxation contributions to axial screw dislocation contrast in synchrotron white-beam X-ray topographs of SiC

Micropipe images appeared in synchrotron white-beam X-ray topographs of thin basal-cut SiC wafers taken using prismatic reflections whereg · b= 0. They consisted of white ovals inclined along the direction of the topographs'gvector that were terminated with dark spikes at either end. The thin wafers tended to curl; the appearance of a defect's image varied depending on the sign of the curvature relative to the side serving as the diffracted beam's exit surface. The micropipe images were computer simulated using the ray-tracing method. The calculation assumed that they arose from the surface-relaxation strain component of a closed-core screw dislocation perpendicular to the surface of a thin foil. The qualitative features of the micropipe images were reproduced in their simulations, but the magnitude of the lattice misorientations predicted by the model was not large enough to account for the size of the experimentally observed dislocation images.

Dislocation image on x-ray topographs within kinematical theory

X-ray topographs of perfect crystals and dislocated crystals have been calculated within kinematical theory. The dislocation image on a x-ray topograph is found to be a circle, a ridge, or a trench depending on the type of dislocation and diffraction condition. Its radius or width is comparable to the radius of the first Fresnel zone, r 1 = λx 0 , where λ is the wavelength of the x rays and x 0 is the distance from the crystal to the observation point. The Fresnel model is applied to the x-ray topograph of solid helium. r 1 is estimated to be 2.8 μm. The different contrast of the subboundaries appearing on the x-ray topographs of solid helium is explained in terms of the contrast of the dislocation images based on the calculation.

Dislocations around precipitates in AlGaN epilayers

Dislocations around precipitates in undoped AlGaN were investigated by transmission electron microscopy. The dislocation images were taken under different diffraction conditions. The dislocations are classified into two types, a pure edge dislocation loop and a close-;coiled helical dislocation. Both types of dislocations were found to depend on the shape and size of the precipitate sources. It is suggested that the pure edge dislocation loop results from homogeneous shear stress and the close-;coiled helical dislocation is caused by spherically symmetrical stress concentration at round ends of the precipitates and chemical force due to defect concentration change.

Contributions of Inelastically Scattered Electrons to Defect Images

We have made measurements of the contribution of inelastically scattered electrons to images of dislocations in Ni{sub 3}Ga and nanometer-sized defects in ion-irradiated Au under weak-beam dark-field diffraction conditions [1]. The purpose is to determine the conditions for data acquisition required to eventually make detailed and quantitative comparisons to simulations of images for various defect models, thus determining defect structure, composition, and local strain field. Such image simulations usually consider only elastically scattered electrons, and thus it is important to understand and possibly eliminate the contribution of inelastically scattered electrons to the experimental images for quantitative comparisons with image simulations. Experimental data have been acquired with either JEOL 2010F or 3000F microscopes, both equipped with Gatan Imaging Filter electron spectrometers. Samples examined in the 2010F were Au, ion-irradiated to low dose (10{sup 11}Kr ions at 1 MeV energy) to form individual defects (1-10nm sized Frank dislocation loops and partial stacking fault tetrahedra). Samples examined in the 3000F were Ni{sub 3}Ga with long dislocation defects. Imaging conditions included weak-beam dark-field with deviation parameter generally > 0.2 nm{sup -1}. Energy filter slit width was set to 10 eV and centered on the zero loss peak in both instruments to obtain images produced by elastically scattered electrons. In the Au experiment an energy window was also set to image the 10-40 eV loss range to investigate the contribution to the defect images of plasmon-loss electrons. Electron intensity in defect images and backgrounds was measured by summing pixel values in appropriately sized rectangles using Digital Micrograph (Gatan) software.

Plane-wave X-ray topography and its application at SPring-8.

Plane-wave X-ray topography experiments were carried out at a 200 m-long beamline, BL20B2, at SPring-8. Relatively high-energy X-rays of 30 keV with an angular divergence of about 0.01 arcsec were produced by using only one collimator crystal. FZ-Si and CZ-Si wafers were characterized in transmission geometry (Laue case). Clear oscillatory profiles in rocking curves of the FZ-Si crystal were observed. Plane-wave topographic images of dislocations, growth striations and grown-in microdefects in the CZ-Si crystals were obtained. The dependence of the topographic images of the lattice defects on the sample-photoplate distance was also studied.

High-resolution phase imaging of phase singularities in the focal region of a lens

Subwavelength-resolution phase images of phase dislocations at the focal region of a 20x , 0.4-N.A. lens have been obtained by use of an optical fiber interferometer with a tapered probe in one arm. A phase-stepping algorithm is used to determine a quantitative value of the phase at each point in the scan, clearly showing the presence of edge dislocations between the Airy rings of the diffraction pattern near the lens focus, as well as four isolated screw-type singularties caused by astigmatism in the lens.

Crystal Structure of and Defects in the Pentacene Thin Film Phase

The aromatic hydrocarbon pentacene is currently under investigation for use as the active layer in electronic devices such as thin film field effect transistors. We have used X-Ray Diffraction (XRD), Electron Diffraction (ED), Low Voltage Electron Microscopy (LVEM), High Resolution Electron Microscopy (HREM) and molecular modeling to investigate the thin film phase of pentacene. We will report the orthorhombic symmetry and lattice parameters of the thin film phase measured experimentally from these techniques. The structure of extended defects such as dislocations and grain boundaries will influence the electrical and mechanical characteristics of the films. Here we show a direct image of an edge dislocation in the thin film phase and discuss the way in which the lattice accommodates the defect.

Synchrotron X-ray topography of undoped VCz GaAs crystals

For the first time vapour pressure controlled Czochralski (VCz) monocrystals of semi-insulating (SI) GaAs, grown at IKZ Berlin, have been investigated by synchrotron X-ray topography. The X-ray topographs of a typical VCz sample, taken from the cylindrical part, show dislocation images resembling those of SI vertical gradient freeze-grown GaAs crystals. From the disappearance of the dislocation image in selected topographs it is concluded that the Burgers vector for most dislocations is parallel to 〈1 1 0〉. The main part proves to be of 60° type. The cellular structure, typical for liquid encapsulated Czochralski material, is not seen in the VCz samples. Large volumes up to 0.5×0.5×0.5 mm 3 are dislocation-free. The results are compared with etch pit density (EPD) measurements from the same crystals. The average EPD is (1–2)×10 4 cm −2. The minimum value along 〈1 1 0〉 is 2×10 3 cm −2.

X-ray topographic dislocation contrast visible in reflections orthogonal to the Burgers vectors of axial screw dislocations in hexagonal silicon carbide

Contrast is associated with micropipes in X-ray topographs of SiC crystals obtained with prismatic reflections, representing an apparent violation of theg·b= 0 invisibility criterion. This is explained as a population of basal-plane dislocations with Burgers vectors of the setb= {\textstyle{1 \over 3}}〈11{\bar{2}}0〉 that occur in a high density within a few micrometers of the micropipes, below the resolution of X-ray topography. These basal-plane dislocations could be observed under an electron microscope. The presence of the surfaces of the micropipes influences the dislocation images in the topographs taken with prismatic reflections, often resulting in a band of light contrast along the axes of the micropipes.