Dislocation Bundles Research Articles

Analysis of synthetic diamond single crystals by X-ray topography and double-crystal diffractometry

Structural features of diamond single crystals synthesized under high pressure and homoepitaxial films grown by chemical vapor deposition (CVD) have been analyzed by double-crystal X-ray diffractometry and topography. The conditions of a diffraction analysis of diamond crystals using Ge monochromators have been optimized. The main structural defects (dislocations, stacking faults, growth striations, second-phase inclusions, etc.) formed during crystal growth have been revealed. The nitrogen concentration in high-pressure/high-temperature (HPHT) diamond substrates is estimated based on X-ray diffraction data. The formation of dislocation bundles at the film-substrate interface in the epitaxial structures has been revealed by plane-wave topography; these dislocations are likely due to the relaxation of elastic macroscopic stresses caused by the lattice mismatch between the substrate and film. The critical thicknesses of plastic relaxation onset in CVD diamond films are calculated. The experimental techniques for studying the real diamond structure in optimizing crystal-growth technology are proven to be highly efficient.

Repulsive interactions between dislocations and overgrown v-shaped defects in epitaxial GaN layers

The spatial distribution and the projected line directions of dislocations intersecting a cross-sectional (101¯0) cleavage plane of a GaN(0001) epitaxial layer is mapped using scanning tunneling microscopy. The data is correlated with the spatial positions of v-shaped defects. The dislocations are found to be bent away from the inclined semipolar facets of v-shaped defects, due to a strain-induced repulsive interaction. The dislocation distribution is characterized by agglomerations and intersecting bundles of dislocations with parallel projected line directions, stabilized by many body effects in the repulsive strain interactions.

Influence of grain size in the near-micrometre regime on the deformation microstructure in aluminium

The effect of grain size on deformation microstructure formation in the near-micrometre grain size regime has been studied using samples of aluminium prepared using a spark plasma sintering technique. Samples in a fully recrystallized grain condition with average grain sizes ranging from 5.2 to 0.8μm have been prepared using this technique. Examination in the transmission electron microscope of these samples after compression at room temperature to approximately 20% reduction reveals that grains larger than 7μm are subdivided by cell block boundaries similar to those observed in coarse-grained samples, with a similar dependency on the crystallographic orientation of the grains. With decreasing grain size down to approx. 1μm there is a gradual transition from cell block structures to cell structures. At even smaller grain sizes of down to approx. 0.5μm the dominant features are dislocation bundles and random dislocations, although at a larger compressive strain of 30% dislocation rotation boundaries may also be found in the interior of grains of this size. A standard 〈110〉 fibre texture is found for all grain sizes, with a decreasing sharpness with decreasing grain size. The structural transitions with decreasing grain size are discussed based on the general principles of grain subdivision by deformation-induced dislocation boundaries and of low-energy dislocation structures as applied to the not hitherto explored near-micrometre grain size regime.

Reconstruction of the Growth History of a Large Synthesized Diamond Crystal by Synchrotron Topography

The growth history of a large diamond crystal synthesized by HPHT temperature gradient method was reconstructed by the defects features in the crystal. The crystal defects were revealed using white synchrotron radiation topography. Dislocations originated from the surfaces of the seed and extended along the directions of <100> were observed within the crystal grown at the early stage. The crystal was then grown to the [00-direction at the middle growing stage and no dislocations were generated at this stage. A lot of dislocation bundles were generated at the last growth stage of the crystal and each bundle was composed of several straight extended dislocations originated from the same point and extended radially within a sector of about thirty degrees centigrade. The origins of the dislocation bundles were distributed near the surface of the crystal. Most of the dislocation bundles were generated near the (-100), (010) and (00-1) and extended to the [-10, [01 and [00-, approximately. The generation of the dislocation bundles was caused by the rapid descent of the temperature in the last growth stage of the diamond crystal.

Investigation of the Crystal Defects in Synthesized Diamond by Synchrotron Radiation Topography

Crystal defects in a large diamond crystal synthesized by high pressure and high temperature (HPHT) method were investigated by synchrotron radiation topography. Five bundles of dislocations were observed in the specimen. Two bundles of the dislocations are in the <100> direction and three of the others are extended at <110>. The Burgers vectors of the dislocations are different within the same bundle as the extinction hinted. The dislocations in <100> direction possess higher energy than the dislocations in <110> and {111}, so they are in the metastable state. The <100> dislocations has not been found in natural diamond yet. The reason of that was the great difference of the growth rate of synthetic and natural diamonds.

Studies on Defect Reduction in AlGaN Heterostructures by Integrating an In-situ SiN Interlayer

We have decreased the dislocation density in AlxGa1-xN epitaxial layers grown on sapphire wafers by introducing an in-situ deposited SiN nano-mask layer. Taking together results obtained by transmission electron microscopy, photoluminescence, cathodoluminescence, and X-ray diffraction, we were able to derive a schematic model about the AlGaN growth on the SiN nanomask: On the open pores of the nano-mask, Ga-rich AlGaN hillocks develop, whereas on the SiN layer Al-rich AlGaN nucleates owing to the reduced selectivity of Al-containing material. Once the hillocks are formed, Ga-rich material is more efficiently incorporated on the inclined side-facets leading to an Al-rich coverage of the central c-plane part of the hillocks. We observed a bending of the dislocations towards the side-facets of the hillocks, which eventually leads to dislocation bundles with increased probability of dislocation annihilation, separated by fairly defect-free regions. Thus, we could achieve a significant reduction of the edge-type dislocation density in these epitaxial layers.

Residual strain around grown‐in defects in CVD diamond single crystals: A 2D and 3D Raman imaging study

AbstractSingle crystal diamond grown by chemical vapor deposition (CVD) sometimes exhibits strain induced birefringence arising from bundles of dislocations lying almost parallel to the [001] growth axis. Some of these specific birefringence patterns consist of four or eight bright petals, depending of the observation conditions. The present study reports results of a Raman imaging investigation of such defects. A specific homoepitaxial film still attached to its substrate, which contained a few of these defects, has been studied. Point by point mapping was used to visualize structural features at certain depths inside the crystals to produce either 2D or 3D tomographic images. These defects were easily detected in the Raman images, with specific line intensity, frequency, and width contrasts. Systematically, dislocations were found to nucleate at the epitaxial interface. In each case, dislocation nucleation was caused by particulate (carbon) contamination. High compressive strain was evidenced in the vicinity of the carbon particles, which was found to decrease in magnitude as the film grew in thickness. Dislocation walls were revealed by the width images. In most cases, 4‐arm patterns lying along 〈110〉 directions were evidenced, but sometimes, more complex features were observed. Finally, polarized Raman measurements gave intensity contrasts that were rather similar to those seen in the birefringence images.

TEM investigations on growth interrupted samples for the correlation of the dislocation propagation and growth mode variations in AlGaN deposited on SiN x interlayers

Threading dislocations (TDs) can be reduced by in-situ deposition of intermediate SiN x sub-monolayers in group III-nitride heterostructures and their ternary alloys. Here we observe efficient dislocation density decreasing at the SiN x nano-mask in an Al x Ga 1− x N layer with x=0.2 grown epitaxially on c-plane sapphire by low pressure MOVPE. However we did not achieve high annihilation efficiency homogenously along the whole SiN x interface in our samples. Areas with high and low annihilation grade alternate along the interface in accordance with respective variations in the SiN x distribution. Furthermore an unusual dislocation bundling was observed for the dislocations in areas with high dislocation densities above the SiN x interface, leading to large areas of several μm 2 with low dislocation densities at the surface between the dislocation bundles. By using growth interrupted samples under same growth conditions, it was possible to investigate the heterostructures in different growth stages by cross-sectional TEM. This enabled us to correlate the dislocation propagation with growth mode variations in AlGaN deposited on the SiN x interlayer and to develop a growth model for the AlGaN layer grown on the SiN x nano-mask.

Impact of type of crystal defects in multicrystalline Si on electrical properties and interaction with impurities

We investigated impact of type of crystal defects in multicrystalline Si (mc-Si) on electrical properties and their change after gettering process of impurities. A bundle of dislocations gives negative impact on the gettering process, while Σ3 grain boundaries does not affect at all. In addition, we categorized random grain boundaries in mc-Si by the contact angle between adjacent dendrite crystals to form the grain boundary. Change in the contrast of photoluminescence intensity around the grain boundary was found to systematically vary by the contact angle, which showed good correlation with calculated interface energy of the grain boundary. Grain boundaries with low interface energy are concluded to be preferable to weaken recombination activity by the gettering process and improvement of solar cell performance based on mc-Si.

Growth Marks of titanian-andradite crystals from Colli Albani (Italy)

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 89-104 - DOI: 10.2451/2011PM0007 Special Issue in memory of Sergio Lucchesi Growth Marks of titanian-andradite crystals from Colli Albani (Italy) Giovanna Agrosi * , Eugenio Scandale and Gioacchino Tempesta Dipartimento Geomineralogico - Universita degli Studi di Bari Aldo Moro, Italy *Corresponding author: g.agrosi@geomin.uniba.it Abstract Growth defects and chemical zonings of Ti-rich andradite (melanite) crystals from Colli Albani were studied using X-ray Diffraction Topography and Electron Probe Microanalysis in order to determine the distinctive features, marks, characterising minerals grown directly from high temperature solution in an open system (volcanic chamber). The samples, cut in (110) and (001) slices, showed a number of primary and, above all, multiphasic melt inclusions, as well as, an unusual colour zoning consisting of a darker colour in the {211} growth sectors with the respect to the {110} sectors. The analysis of X-ray topographic contrasts of growth defects as growth bands, sector boundaries and bundles of dislocations developed parallel to the growth directions has enabled the reconstruction of the morphological evolution vs. time of the samples. The results also suggest that from their onset the crystals were faceted with {211} and predominant {110} faces. Moreover, the topographic images indicated that most of the examined dislocations were nucleated from inclusions. By using the extinction criterion, it was possible to determine that these defects are characterised by a strong edge component. The faceted morphology of garnets found in this paper and the fact that edge dislocations were only characterised suggest a layer spreading growth mechanism by two-dimensional nucleation. The reconstruction of morphology was also particularly useful in distinguishing two different types of chemical zoning: concentric and sector zoning. The results of chemical analyses confirmed that the optically observed colour zoning coincided with sector zoning because of a small but meaningful enrichment of TiO 2 and a decrease of Al 2 O 3 in correspondence with {211} growth sectors. Instead, concentration variations, chiefly of TiO 2 and SiO 2 , were recorded crossing successive growth stages. This event appeared to be associated with the physio-chemical evolution of the growth environment and was defined as concentric zoning. Comparing the results found in our study with those of previous studies on garnets from hydrothermal and metasomatic environments, the meaning of the growth marks in the Ti-rich andraditic garnets from Colli Albani could be assigned to primary multiphase melt inclusions, concentric and sector zoning, sector boundaries, growth bands and edge dislocations since these defects characterise and distinguish our samples from those grown in different genetic environments. Key words : X-ray Topography; Ti-rich andradite; structural defects; concentric zoning; sector zoning.

High quality AlGaN epilayers grown on sapphire using SiN x interlayers

We have investigated the optimization of Al 0.2Ga 0.8N layers directly grown on sapphire by metalorganic vapor phase epitaxy (MOVPE). The quality of the AlGaN epilayers was improved by in situ nano-masking employing ultra-thin SiN x interlayers. Transmission electron microscopy (TEM) investigations reveal an enormous reduction of edge-type dislocations by SiN x nano-masking. Furthermore, formation of bundles of dislocations converging to the surface is visible, leading to large areas up to 1 μ m in size on the surface which are almost defect free. The SiN x surface coverage was carefully optimized resulting in narrower (1 0 2)-reflections of high resolution X-ray diffraction (HRXRD), down to full width at half maximum (FWHM) values of 570 arcsec, in addition to narrow symmetric HRXRD reflections down to FWHM of 150 arcsec. Structures with double SiN x interlayer revealed even higher quality of the epilayers with a FWHM of 440 arcsec for the (1 0 2)-reflection. A series of GaN–AlGaN multi-quantum wells were grown on such high quality templates. The MQWs show a significant decrease in peak widths and also an increase in the luminescence intensity. Furthermore, these templates have been used as buffer layers for ultraviolet light-emitting diodes emitting at 350 nm.

Theory of the birefringence due to dislocations in single crystal CVD diamond

Single crystal diamond grown by chemical vapour deposition (CVD) often exhibits straininduced birefringence arising from bundles of edge dislocations lying almost parallel to the[001] growth axis. The birefringent pattern changes when the crossed-polarizersare rotated with respect to the underlying lattice. For polarizers parallel to⟨110⟩ directions, the birefringence pattern consists of four bright petals with dark arms along⟨110⟩. For polarizersparallel to ⟨100⟩, the birefringence pattern consists of eight petals of weaker intensity with dark arms along⟨110⟩ and⟨100⟩ directions. We evaluate the birefringence intensity by using isotropic elasticity theory andfind that these patterns can be explained by a specific dislocation arrangement which isconsistent with x-ray topographic studies.

Morphology of {100} and {201} faces and the optical quality of KTiOPO4 crystals

Potassium titanyl phosphate crystals up to 350 g in weight have been grown from a high-temperature solution in K4P2O7 by the flux method on a [100]-oriented seed. The micromorphology of the (201), (20\( \bar 1 \)), and (100) faces is studied, and the relationship of the growth accessories on these faces with optical inhomogeneities in the crystal bulk is established. It is found that the highest rate of tangential step growth on these faces is observed in the [010] direction. The growth centers on the pinacoid front face are related to the dislocation bundles arising at seed regeneration or during defect growth of {110} and {011} faces. In the absence of dislocation growth centers on the (20\( \bar 1 \)) and (201) faces, the sources of growth striations are edges formed by neighboring faces and the lateral boule surface.

Cyclic Softening Behaviour of a Duplex Stainless Steel

Abstract This first part aims to study the correlation between the cyclic softening observed during low-cycle fatigue in a 2507 duplex stainless steel (DSS) alloyed with 0.17% N and the dislocation structure development. Tension-compression and tension-compression- torsion fatigue tests were carried out under total strain control at Δεteq/2 = 0.5%. The dislocation structures were characterized by transmission electron microscopy during cycling with interrupted tests. The austenite showed planar arrangements of dislocations during all the fatigue life, the ferrite showed evolution from a homogeneous distribution of dislocations to a structure of dislocation bundles in tension-compression, and to a refined wall structure in tension-compression-torsion. It is proposed that the evolution of the dislocation structure in the ferrite plays an important role during the cyclic softening in the DSS.

Effect of γ 2-phase precipitates on the martensitic transformation of a β-CuAlBe shape memory alloy

The influence of γ 2 precipitates on the MT of a Cu–22.66Al–2.98Be (at%) polycrystalline alloy was analysed by compression test, differential scanning calorimetry and microscopy. As the precipitate content (or size) increases, the stress-induced martensitic transformation occurs at higher stress levels. The thermally induced martensitic transformation is only significantly affected when the γ 2 phase content exceeds 14%, the thermal peaks are shifted to higher temperatures and less β phase transforms to martensite. TEM observations suggest good coherency between both phases, and dislocation bundles remain in the β side of the γ 2/β interface after the compression-induced transformation to martensite. The evolution of σ s with fvγ 2 is analysed considering a chemical and a mechanical contribution.

Growth and Characterizations of GaN-Based LEDs Grown on Wet-Etched Stripe-Patterned Sapphire Substrates

The material and electrical properties of GaN-based light-emitting diodes (LEDs) grown on wet-etched stripe-patterned substrates were investigated. Footprint-like patterns, located directly above the inclined groove sidewalls, were found on the as-grown LED surface. Cross-sectional transmission electron microscopy (TEM) showed that ‘tumor’-like structures with poor crystal quality were initiated on the inclined sidewalls, seeding dislocation bundles in the subsequently grown crystal. The high dislocation density slowed down the growth above the inclined sidewall, resulting in the uneven morphology. The fabricated devices showed over 30% enhancement in light output power as a result of improvements in both internal and extraction efficiencies.

Stresses to bow edge dislocation segments out of di-/multipolar edge dislocation bundles

In deformed crystalline materials, edge dislocation segments bowing out of di-/multipolar edge dislocation bundles and walls are frequently observed. In the present work, a simple estimate is given of the stress that must be applied in order to detach such bowing-out edge dislocation segments from the bundles. Instead of dealing with this problem in its whole complexity, a strongly simplified situation is considered in which the bowing-out segments interact with only one closely neighbouring edge dislocation of the same or of the opposite sign. In the calculations, only the elastic interaction between the two dislocations and the restoring force acting on the bowing-out dislocation segment due to the line tension are considered. The calculations were performed for different combinations of dislocation segment lengths and spacings of the glide planes on which the two dislocations considered lie. The results obtained for the detachment stresses are compared and discussed.

Dislocation Reduction Mechanism in Low-Nucleation-Density GaN Growth Using AlN Templates

Achieving a high output power and a high energy efficiency in ultraviolet light-emitting devices requires a reduction in the dislocation density of an underlying GaN template. Conventional thin film growth is a two-step process that starts with a low-temperature buffer layer (also known as the wetting or nucleation layer), followed by the growth of high-temperature GaN islands that eventually coalesce and planarize to form the basic template for device structures. We have studied these initial growth stages and showed that reducing the density of GaN islands contributes to the reduction in dislocation density. However, an excessive reduction in island density leads to the worsening of film surface morphology, with the appearance of pits and spiral features. We have observed that dislocation density is related to the crystal tilts of GaN nucleation islands and that pits and spirals are generated from the screw-type dislocation bundles formed at low GaN island densities. The alternative use of AlN templates (instead of conventional GaN buffer layers) consequently improves the c-axis crystal orientation and reduces the threading dislocation density in the GaN films while simultaneously providing a high-quality surface morphology.

Pseudoelasticity in Fe<sub>3</sub>Al Single Crystals under Cyclic Loading

D03-ordered Fe3Al single crystals containing 23, 25 and 28at.%Al were cyclically deformed at [ 49 1 ] loading axis in tension-compression mode. Giant pseudoelasticity took place in Fe-23 and 25at.%Al single crystals at an early stage of cyclic deformation, while a slight strain recovery was observed in Fe-28at.%Al. In Fe-23 and 25at.%Al single crystals, superpartial dislocations with Burgers vector of 1/4[111] moved individually dragging the nearest neighbour anti-phase boundary (NNAPB). The NNAPB pulled back the superpartials during unloading, resulting in giant pseudoelasticity and low residual dislocation density. In contrast, a couplet of the superpartials was observed to bow out leaving two superpartials in Fe-28at.%Al. This means that the dislocation couplet dragged the next-nearest neighbour anti-phase boundary (NNNAPB). The surface tension of NNNAPB is lower than that of NNAPB resulting in a slight strain recovery in Fe-28at%Al. As cyclic deformation proceeded, residual dislocation density increased with an increase in the number of cycles even in Fe-23 and 25at.%Al. In particular, persistent slip bands (PSB) were formed in Fe-23at.%Al single crystals, though PSB is seldom observed in fatigued intermetallic compounds. To-and-fro motion of superpartials during loading and unloading was suppressed by dislocation bundles, resulting in a reduced shape recovery. However, large strain recovery occurred in Fe-25at.%Al single crystals at a strain amplitude of 1.0% even at 20cycles. It was also noted that Fe-23 and 25at.%Al demonstrated tension-compression asymmetry even at [ 49 1 ] orientation; the yield stress in compression was higher than that in tension. This implies that the core structure of <111> screw dislocation played an important role in the deformation behaviour of Fe3Al single crystals.

SiGe/Si layers—early stages of plastic relaxation

The plastic relaxation of SiGe/Si is closely related to the nucleation of misfit dislocations at early stages. We have investigated the very early stages at annealing temperatures ranging from 520°C to 670°C by means of x-ray topography (XRT), atomic force microscopy (AFM) and transmission electron microscopy. At misfit dislocation densities within the range accessible by XRT, i.e. up to about 1000 cm−1, dislocations bundles predominate. This is verified by AFM and is explained by heterogeneous nucleation. The fewer the dislocations that are contained within dislocation bundles the rarer the blocking and cross slip that are observed. It is demonstrated that laser heating increases the number of nucleation centres drastically and is well suited to induce nucleation at selected sites. Furthermore, these nucleation centres provide dislocation bundles containing only a few dislocations. In contrast to this, implantation can produce defects that stop dislocation propagation quite effectively at the comparatively low temperatures used in our experiments.