Dislocation Generation Mechanisms Research Articles

Dislocation-grain boundary interactions in ice crystals

Abstract Dislocation-grain-boundary (GB) interactions in polycrystalline ice Ih during creep have been studied in situ using synchrotron X-ray topography. The basal slip system with the highest Schmid factor was found to be the most active in polycrystalline ice whereas the GB orientation relative to the loading direction seemed unimportant. GBs act both as effective sources of lattice dislocations and as strong obstacles to dislocation motion. The observations revealed pile-up formation upon loading and pile-up relaxation after unloading. Non-basal segments of lattice dislocations can be generated from GBs in ice. However, they neither noticeably decrease stress concentrations nor contribute significantly to the overall plastic deformation. It was found that dislocations can be generated from both free-surface-GB intersections and from the interiors of GBs, indicating that the dislocation generation mechanism presented in our 1993 paper is not a surface artefact. Evidence is also presented that, because ...

Misfit dislocation generation mechanisms in InGaAs/GaAs heterostructures

An experimental investigation of misfit dislocation generation mechanisms at an InGaAs/GaAs heterointerface is reported. InGaAs epitaxial layers were grown by low-pressure organometallic vapor-phase epitaxy on patterned and unpatterned GaAs substrates having etch-pit densities (EPD) of 200, 1400, and 10 000 cm−2. After epitaxial growth, the samples were annealed at temperatures between 650 and 750 °C, and analyzed by optical and transmission electron microscopy. For the range of substrate EPD studied, it was found that the substrate EPD controls the onset of misfit dislocation generation for low-temperature epitaxy (<600 °C) on unpatterned substrates. When epilayers were annealed at 750 °C, the density of misfit dislocations was independent of the substrate EPD. These studies also show that the dominant misfit dislocation generation mechanism for films grown on patterned substrates is nucleation at the growth-mesa edge. The density of preexisting threading dislocations has little influence on misfit dislocation generation for films selectively deposited within 100×100 μm2 growth windows. For selective heteroepitaxy, misfit dislocation generation strongly depends on the crystallographic orientation of the growth-mesa edge.

High-resolution transmission electron microscopy characterization of III–V compounds on Si grown by metalorganic chemical vapor deposition

Misfit dislocations in GaAs, AlGaAs and GaP layers grown on Si substrates by metalorganic chemical vapor depositions are characterized by high-resolution transmission electron microscopy (HRTEM). The lattice mismatch of GaAs on Si is completely relaxed by the misfit dislocations. The Burgers vector of the misfit dislocation is greatly affected by the growth mode at the initial stage of the growth. Lomer misfit dislocations are dominant when the epitaxial layer grows three-dimensionally at the initial stage (GaAs on Si and AlGaAs on Si), and 60° dislocations are dominant when the epitaxial layer grows two-dimensionally at the initial stage (GaP on Si). A mechanism for Lomer misfit dislocation generation in the case of three-dimensional growth is proposed.

Structural properties of CdMgTe/CdTe superlattices

CdMgTe/CdTe superlattices with a high degree of structural perfection were grown on CdTe and Cd0.975Zn0.025Te substrates by molecular-beam epitaxy. The structural properties of the superlattices, i.e., the morphology of the layers and the state of strain relaxation, were examined by transmission electron microscopy and x-ray diffractometry. High-resolution transmission electron microscopy (HRTEM) reveals the structural quality of the superlattices on an atomic scale. The width of the chemical transition between the CdTe and CdMgTe layers was determined by HRTEM using chemically sensitive imaging conditions. Two different mechanisms of misfit dislocation generation could be observed in situ in the electron microscope studying a cross-section specimen of a superlattice which was originally fully strained.

Studies of Defect Behavior in Large-Grain, Polycrystalline Ice Using Synchrotron X-ray Topography

Abstract Synchrotron White Beam X-ray Topography (SWBXT) has been used to conduct in situ, low temperature studies of the behavior of defects introduced into large-grain, polycrystalline ice of very low “as-grown” defect density. The generation of faulted and unfaulted interstitial dislocation loops as a function of imposed temperature changes was observed. Variations in the distribution of these loops in the vicinity of grain boundaries are discussed in the context of diffusion mobilities on the basal plane and the relative orientation between the basal plane and the grain boundary plane. Dislocation generation mechanisms under applied compressive stresses were also investigated in situ using a specially-designed compression stage. This has led to the novel observation of dislocation nucleation at stress concentrations on grain boundaries. The utility of SWBXT in dynamic studies of this general nature is demonstrated.

Dislocation generation mechanisms of InxGa1−xAs (0≤x≤1) epilayers grown on (100) InP substrates by molecular beam epitaxy

The mechanisms of strain relaxation and dislocation generation for the 2-μm-thick InxGa1−xAs epilayers grown on (100) InP substrates with 0≤x≤1 were investigated. It was found that the growth mode and dislocation density of the InxGa1−xAs epilayers are not only dependent on the lattice mismatch with respect to InP substrates, but the abundance of Ga atoms and the degree of cation disorder in the alloy composition also play important roles. In the negative mismatched range even with a medium lattice mismatch (e.g., ε=−1.1%), InGaAs alloys with a high degree of cation disorder and containing more Ga atoms (x=0.32–0.37) trigger island growth and introduce high-density V-shaped dislocations. In the positive mismatched range, island growth occurs at x≊0.82 (ε=2%) and few V-shaped dislocations are generated. The difference between these two ranges is due to their different Ga concentrations which introduce different island nucleation centers in the initial growth stage.

Experimental Modeling for the Compressive Strengthening Mechanism of an Al-Li / SiCp Composite

The main strengthening mechanism for a SiCp / Al-Li alloy composite has been compared with a dislocation generation mechanism, under compression loading. Both higher volume fraction and a smaller SiC particle size was found to give higher increases in the compressive yield stress. However, the results also indicate that particulate reinforcement could only give limited increases in the yield stress.

Characterization of Defect Structures in Lely 6H-SiC Single Crystals Using Synchrotron White Beam X-Ray Topography

AbstractDefect structures in Lely SiC single crystals have been studied using synchrotron white beam X-ray topography. Basal plane dislocations and stacking faults probably generated during post-growth cooling are clearly revealed. For both perfect dislocations and partial dislocations bounding the stacking faults, Burgers vectors and line directions are determined from contrast extinction analysis as well as projected direction analysis on different topographic images. The fault planes and fault vectors of the stacking faults were determined using contrast extinction analysis. Possible dislocation generation mechanisms are briefly discussed.

Dislocation generation mechanisms for GaP on Si grown by metalorganic chemical vapor deposition

Dislocation generation mechanisms for GaP on Si substrates by metalorganic chemical vapor deposition are described. Dislocations are not observed at the GaP/Si interface when the layer thickness is less than 90 nm. The presented high resolution transmission electron microscopy shows two kinds of dislocations with the extra-half plane in the GaP layer and Si substrate. These observations predict that the misfit dislocations are formed at the growth temperature while the dislocations with the extra-half plane in the GaP layer are formed during the cooling process, owing to the difference of the thermal expansion.

The growth of highly mismatched InxGa1−xAs (0.28≤x≤1) on GaAs by molecular-beam epitaxy

The material quality of InxGa1−xAs epilayers with 0.28≤x≤1 grown on a highly mismatched GaAs substrate and the initial growth mechanisms have been studied in detail. Cross-sectional transmission electron microscopy and double-crystal x-ray diffraction analysis of the InxGa1−xAs (x≥0.28) layers indicate that although three-dimensional island growth was found in all cases, their dislocation generation mechanisms are quite different. To study the strain relaxation process in the initial growth stage and its effects on the generation of the dislocations, 15-monolayer-thick InxGa1−xAs was grown on the GaAs. It was found that for the x=0.5 case, the thin InGaAs layer formed many small-size coherent islands; while for x=1, they changed to lower-density and much larger size relaxed islands. This shows that the growth of the highly mismatched InxGa1−xAs/GaAs heterojunction (x≥0.28) is a kinetically limited strain relaxation process. The large lattice mismatch (7.16%) between InAs and GaAs favors the early relaxation of InAs islands before they coalesce; this explains why the material quality of the InAs epilayer is better than that of InxGa1−xAs with x=0.28, 0.5, and 0.75 in spite of its larger lattice mismatch.

Dynamic observations of dislocation generation at grain boundaries in ice

Dynamic in situ X-ray topographic deformation studies have been performed on polycrystalline ice. Based on these observations, a new and dominant mechanism for dislocation nucleation, which is related to stress concentrations observed in the vicinity of grain boundaries, is proposed. It was found that the areas near grain boundaries always deform before the grain interiors. Lattice dislocations were nucleated continuously at large-angle grain boundaries, driven by internal stresses that are higher than the external stress. The dislocations, once generated, glide on the basal plane as semi-hexagonal loops. The shape of these loops is a result of a balance of the stresses present. The dislocation generation mechanism at grain boundaries was found to depend strongly on the basal plane orientation relative to both the loading direction and the grain boundary plane.

Pressure-induced dislocations and subsequent flow in NiAl

The effects of pressurization on the generation of dislocations and their effects on the subsequent flow behavior of cast, extruded and annealed NiAl are described. It is shown that simple pressurization to either 500 or 1400 MPa produces dislocation generation both at second phase particles and at grain boundaries in certain grains. The pressure-induced dislocations are of 100 type, while the pressurized specimens exhibited significantly lower initial flow stresses at 0.1 MPa in comparison to the as-annealed NiAl. Possible mechanisms of dislocation generation are described.

Dynamic observation of dislocation sources at grain boundaries in ice

Abstract This Letter presents the first clear demonstration of a grain-boundary dislocation generation mechanism in ice. The dislocations generated as semihexa-gonal loops were observed to glide on the basal plane.

Dislocations and precipitates in gallium arsenide

A complete dislocation analysis on a large number of grown-in dislocations was performed on wafers taken from three different semi-insulating liquid encapsulation Czochralski GaAs single crystals. By determining the Burgers vector, line direction, and habit plane of nearly 800 dislocations a decision could be made on dislocation type, dislocation generation, and multiplication mechanisms. Taking into account possible dislocation reactions between stress-induced glide systems all detected glide systems could be explained. The influence of post-growth annealing on both dislocations and arsenic precipitates was also investigated. Little effect was found on dislocations. Arsenic precipitates, however, showed a different distribution in size and significant effects on fine structure giving information on their nucleation and growth mechanisms.

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.

Misfit Dislocation Nucleation and Interactions at GexSi1-x/Si Interfaces

AbstractIn the study of elastic strain relaxation in semiconductor heterostructures, a number of misfit dislocation generation mechanisms have been suggested to account for the high interfacial dislocation density observed in these almost defect-free crystals. Several MBE-grown GexSi1-x/Si heterostructures, both in the as-grown and annealed condition have been studied using transmission electron microscopy. The results indicate that some of the popular theories of dislocation generation are less important or not applicable based on both theoretical and experimental considerations. Specifically, it will be shown that: (i) heterogeneous sources play a dominant role in the nucleation mechanisms, (ii) the strain relaxation behaviour during MBE growth may be different from that observed in metastable structures annealed after growth and (iii) the Hagen-S trunk multiplication mechanism is inoperative under most conditions in this system.

In situsynchrotron X-ray topography study of the generation of lattice dislocations in aluminium by migrating grain boundaries

Abstract The generation of lattice dislocations by migrating grain boundaries has been studied in aluminium, both in situ and in real time, by synchrotron white-beam X-ray topography (SWBXT). It has been established that matrix screw dislocations are left by, and trailed behind, moving grain boundaries and that these defects are incorporated in a dislocation network produced by reaction with dislocations originating from the grain nucleus. As opposed to Gleiter's findings in InP and InAs, the density of matrix dislocations left by a moving grain boundary in aluminium does not depend on its migration rate. The production of these defects seems better associated with stresses acting in the grain boundary plane during the recrystallization process, which leads to a proposal for a new dislocation generation mechanism.

Dislocation Generation Mechanisms and Layer Growth in InP Epitaxy as a Function of Growth Conditions

Dislocation Generation Mechanisms and Layer Growth in InP Epitaxy as a Function of Growth Conditions

Substructure development in shock-loaded Cu-8.7 Ge and copper: the role of temperature, grain size and stacking fault energy

The effect of grain size and deformation temperature on the shock-hardening response and substructure development in Cu-8.7 Ge (where the composition is in atomic per cent) has been studied, as has the behavior of copper shocked at ultrashort pulse durations. In all cases, shocking introduced deformation twins, which formed initially as bundles of finer twins and then coalesced to form more perfect twins. Decreasing the deformation temperature did not significantly affect the shock-loading response of Cu-8.7 Ge. Increasing the grain size resulted in decreased dislocation and twin generation at short pulse durations. At long pulse durations, twin thickening dominated the deformation processes. Deformation twins were observed in copper at shock pulse durations as short as 20 ns, with the twin bundles being much thicker than those in Cu-8.7 Ge. The dislocation generation rates as a function of strain were found to be consistent with those for conventional deformation, indicating that the dislocation generation and multiplication mechanisms are the same for shock-loading and conventional deformation.

Generation mechanism of misfit dislocations in In1− xGaxAs1− yPy/ InPDH structure grown by LPE

Misfit dislocations in InP/InGaAsP/InP DH wafers have been investigated by X-ray transmission topography in relation to lattice mismatch measured by a double crystal X-ray diffractometer. The misfit dislocations are 60° dislocations and are generated in the top InP layer around the wafer periphery. The negative lattice mismatch between the top InP layer and the substrate is always observed in the region with misfit dislocations and is not observed in the region without misfit dislocations. With heat-treatment, most of the misfit dislocations are unaffected and some new ones are generated. The following generation mechanism is proposed: mobile 60° dislocations rapidly move toward the center region from dislocation sources at the wafer edges in the top InP layer during growth due to the lattice mismatch berween the top InP layer and the substrate and they move more easily in InP than in InGaAsP.