Dynamic Aspects of Dislocation Behaviour Near Low-Angle Boundary in Molybdenum Single Crystals

The dislocation behaviour was inventigated in specimens of monocrystals of molybdenium during in — situ stretching in the direction [001] near low — angle twist boundary which was close to be parallel to the direction of the external force. Low-angle boundary is easily penetrable for the fast moving nonscrew dislocations and acts as an effective stopper for screw dislocations which are quite parallel to the forming boundary dislocations. The mixed tipe dislocations emission by the low-angle boundary was noticed. On the stage when plastic deformation is performed mainly by the motion of screw dislocations the “relay-race”-like transmission of the dislocations motion through the boundary was observed.     

Propagation behavior of threading dislocations during physical vapor transport growth of silicon carbide (SiC) single crystals

The dislocation formation and propagation processes in physical vapor transport (PVT) grown 4H silicon carbide (4H–SiC) single crystals have been investigated using defect selective etching and transmission electron microscopy (TEM). It was found that while the growth initiation process generally increased the density of threading dislocations in the grown crystal, for certain areas of the crystal, threading dislocations were terminated at the growth initiation. Foreign polytype inclusions also introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, almost no medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the foreign polytype inclusions had ceased the propagation of threading screw dislocations. Based on these results, we argued the formation and propagation of the threading dislocations in PVT grown SiC crystals, and proposed the dislocation conversion process as a plausible cause of the density reduction of threading dislocations during the PVT growth of SiC single crystals.     

The role of dislocations in the formation of mechanical stresses during annealing of gallium arsenide single crystals

The effect of dislocations on the change of mechanical stresses in undoped semi-insulating gallium arsenide single crystals has been studied during their annealing in vacuum and in the arsenic atmosphere. The phenomena observed are explained by the effect of dislocations playing the role of channels for arsenic diffusion on the concentration of intrinsic point defects in the crystal regions surrounding dislocations. The mechanism of arsenic diffusion over dislocations allows one to consider dislocations as sources and sinks of arsenic without the translational and twinning processes and, thus, makes the well-known data on the reactions of dislocation interaction with point defects and the experimental structural data for single crystals more consistent.     

Study of the Nature and Distribution of Elementary Steps on the (100) Cleavage Faces of MgO Single Crystals by Atomic Force Microscopy

Some observations made on the nature and distribution of monolayer (elementary) steps on the (100) cleavage faces of MgO single crystals by atomic force microscopy are presented and discussed. The following types of patterns of monolayer steps are described: (1) trains of steps, (2) steps terminating on the cleaved surface at the emergence points of screw dislocations, and (3) localized pinning of advancing steps at random sites (probably at the emergence points of edge dislocations). It is shown that: (1) the origins of emergence points of monolayer steps are devoid of hollow cores due to a small Burgers vector of dislocations and (2) the minimum distance between two emerging steps due to screw dislocations and between two pinning centres due to edge dislocations depends on their sign, and is determined by the mutual interaction between neighbouring dislocations.     

Dislocation mobility in LiF at low temperatures

The mobility of dislocations in LiF has been measured at 4.2 and 10 K by means of an etch pit technique. The average velocity of screw dislocations is 2–5 times as large as edge dislocations. The results of the stress and temperature dependence of the velocities correlate well with the data of the critical shear stress in the same temperature region, and are well described by the theory of Peierls mechanism with thd Peierls stress of about 20 MPa for edge dislocations on {110} slip plane.     

Characterization of defects in GaP and GaAsP graded heterojunctions by transmission electron microscopy

Misfit dislocations are observed in graded heterojunctions GaAs_1?xP_x by electron microscopy. Results are in agreement with previous work concerning the nature of the dislocations (Lomer and 60° dislocations) and their density dependence on the phosphorus gradient. The discussion concerns the formation of Lomer dislocations and the possibility of reducing the density of inclined dislocations which reach the surface of the epitaxial layer. GaP substrates, S-doped, are examined by transmission electron microscopy. Numerous defects such as Frank loops, perfect loops, helical dislocations and precipitates are observed. A GaP homoepitaxial layer realized on this substrate is free from these defects but exhibits stacking faults. A zinc diffusion does not produce additional defect but a 1000 Å thick amorphous layer is observed a at the surface.     

On the slope of the growing pyramids

By means of some theoretical models the present work shows that pyramids of different slopes can grow on a crystal face at constant supersaturation due to the interaction between steps, arising from close spaced screw dislocations. Pairs of adjacent screw dislocations of opposite sign, as well as other groups of closely spaced screw dislocations, can operate as more active sources of growing steps than the single screw dislocations. Both independent and dependent step propagation velocities have been considered in the present discussion.     

Positron annihilation study of defects in martensite transformation of Fe?Mn alloy

It has been observed that a small number of repetitions of the transformation by thermal cycling enhanced the formation of ϵ-Martensite whereas a large number of repetitions of transformation decreased the amount of ϵ-Martensite formed. PAT analysis and electron microscope show that a few repetitions of the transformation produced less defects, including vacancies and dislocations, whereas a large number of repetitions of transformation produced progressively more defects. The PAT analysis also shows that these accumulated defetcs are mainly dislocations, which form a network of dislocations or a Lomer-Cottrell dislocation net. A small number of dislocations enhanced the nucleation of ϵ-Martensite whereas a large number of dislocations prevented the nucleation. So the dislocations-produced by cycling transformation thermally — has a great effect on the ϵ-formation.     

Röntgentopographische Untersuchung von KDP-Kristallen

The dislocation configuration arising in KDP crystals grown from the solution has been studied by X-ray diffraction topography method (Lang-method). The space orientation of dislocations and their Burgersvektor directions are determinated. The dislocations density did not exceed 10³ cm⁻², and there are areas which were free form dislocations. The sources of dislocations were classified as inclusions of mother liquor and foreign particles respectively.     

Zum Nachweis von Versetzungen in β-Kupferphthalocyanin-Einkristallen

An etching technique was developed to investigate dislocations in β-copper phthalocyanine single crystals. Considering the expected content of dislocations and the etch pit symmetry the symmetric etch pits are correlated to [010]-edge dislocations on {201 }- and {001}-lattice planes with (001)- and (201 )-slip planes. Asymmetric etch pits on {001}- and {201 }-planes are connected with [010]-edge dislocations related to (100)- and (101 )-slip planes. The dislocation density on growth planes and cleavage planes is commonly lower than 100 cm⁻². [010]-screw dislocations are not observed, but their existence could not be excluded.     

Lattice strain and misfit dislocations in GaAs-GaAlAsP heterojunctions

The lattice strain and misfit dislocations in a GaAs-GaAlAsP heterojunction were examined. The change in lattice strain with the composition ratio or the position of the crystal was measured, and it was found that some of the misfit dislocations introduced in the heterojunction were edge-type dislocations. The critical thickness of the epitaxial layer for the generation of misfit dislocations was also measured. The critical differential strain between the atomic layers was on the order of several angstroms. The distribution of lattice strain was analyzed by a two-dimensional simple cubic lattice model, and the distribution of differential strain was examined.     

Dislocations in synthetic quartz

The present paper is a review of the data about the dislocations in the as-grown quartz crystal. The work presents some general considerations about dislocations, their origins and causes, methods of their observation and techniques to reduce the dislocations density. It is also shown a method to obtain a new shape crystal used in the positive X-seeds preparation.     

Influence of non-stoichiometry on the recombination activity of dislocations in undoped semi-insulating GaAs crystals

The effect of non-stoichiometry of gallium arsenide melt composition on the recombination activity of dislocations in undoped semi-insulating GaAs crystals is studied and analyzed. It is shown that the deviation from the stoichiometric melt composition is one of the major factors affecting the recombination activity of dislocations in undoped semi-insulating GaAs crystals. namely: the recombination activity of dislocations is increased when the arsenic-rich melt is used, and, on the contrary, is decreased when the gallium-rich melt is used. The regularities observed are explained by the complex processes of interaction of dislocations with the non-stoichiometry-induced intrinsic point defects.     

Misfit Strain Relaxation by Dislocations in InAs Islands and Layers Epitaxially Grown on (001)GaAs Substrates by MOVPE

The misfit dislocation configurations in InAs islands as well as in more or less continuous layers grown on (001) oriented GaAs substrates were studied by weak-beam and high-resolution electron microscopy. The islands are confined by {101} and {111} facets where the aspect ratio (height/lateral extension) can be affected by the growth conditions. It is possible to grow well-defined islands as well as relatively continuous layers by MOVPE under As-stabilized conditions. At constant deposition parameters the growth is characterized by islands of different sizes (but with constant aspect ratio) in various strain states depending on their dislocation content. Coherently strained islands without any dislocation can be observed for heights up to 23 ML InAs, or otherwise, up to a maximal island extension of about 12 nm (for the particular aspect ratio ≈︂0.585). With further increase of island height and lateral extension, the introduction of dislocations becomes favourable. Independent of the island size, the layer thickness and the dislocation density, a residual elastic strain of about ε^r = —0.8% remains after relaxation. This means, about 88% of the total misfit strain of ε = —6.686 × 10^—2 were compensated by Lomer dislocations. These sessile Lomer dislocations lie in the island interior only, where single 60° dislocations were observed exclusively in their near-edge regions. With increasing island size and/or layer thickness some close-spaced 60° dislocations occur additionally within the interfacial region. The Lomer dislocations that are always located 4 monolayers (ML) above the InAs/GaAs interfacial plane result from the well-known fusion of two 60° slip dislocations. These 60° dislocations have been nucleated 7 … 8 ML above the interface at surface steps on the {111} facets confining the islands. Based on our experimental observations a new mechanism is proposed that explains the origin of these 60° dislocations. Their further fusion to sessile Lomer dislocations that compensate the misfit strain most efficiently occurs in the way as commonly accepted.     

Calculation of displacement fields and simulation of HRTEM images of dislocations in sphalerite type A(III)B(V) compound semiconductors

The displacement fields of different kinds of both perfect and dissociated dislocations have been calculated for an isotropic continuum, and by means of linear elasticity. Additionally, the corresponding HRTEM images have been simulated by the well-established EMS program package in order to predetermine the structural aspects of dislocations, and then to compare it with experimental HRTEM micrographs. The latter ones resulted from plastically deformed GaP single crystals and InAs/(001)GaAs single epitaxial layers. It could be established that using the simple approach of linear elasticity and isotropy results can be obtained which correspond well to the experimental images. So, the structure of various Shockley partial dislocations bounding a stacking fault can be detected unambiguously. The splitting behaviour of perfect 30° dislocations (separation into a 0° and 60° partial) and 90° dislocations (separation into two 60° partials) both with line direction along 〈112〉, 60° dislocations (separation into 30°/90° and 90°/30° configuration) and screw dislocations (separation into two 30° partials) along 〈110〉 are discussed in the more detail. Moreover, the undissociated sessile Lomer dislocation, glissile 60° dislocation and edge dislocation have been considered too.     

Diffusion of self-point defects in body-centered cubic iron crystal containing dislocations

The energetic, crystallographic, and diffusion characteristics of self-point defects (SPDs) (vacancies and self-interstitial atoms (SIAs)) in body-centered cubic (bcc) iron crystal in the absence of stress fields have been obtained by the molecular statics and molecular dynamics methods. The effect of elastic stress fields of dislocations on the characteristics of SPDs (elastic dipoles) has been calculated by the methods of the anisotropic linear theory of elasticity. The SPD diffusion in the elastic fields of edge and screw dislocations (with Burgers vectors 1/2 〈111〉 and 〈100〉) at 293 K has been studied by the kinetic Monte Carlo method. The values of the SPD sink strength of dislocations of different types are obtained. Dislocations are more effective sinks for SIAs than for vacancies. The difference in the sink strengths for SIAs and vacancies in the case of edge dislocations is larger than the screw dislocations.     

Luminescence during release of pressure in X-ray irradiated alkali halide crystals

Emission of light is found during application as well as release of uniaxial pressure in X-ray irradiated alkali halide crystals. As unpinning of dislocations can not take place, however, dislocations can move in backward directions during release of the pressure; the emission of light during release of pressure gives a direct support that the interaction of moving dislocations with F-centers is responsible for the emission of light during deformation of colored alkali halide crystals.     

Partial dislocations and stacking faults in cubic SiC

Numerous stacking faults and dislocations (formed by intersection of stacking faults and dislocations limiting nonintersecting stacking faults) in the 3C-SiC films grown by molecular beam epitaxy on a silicon substrate were studied by electron microscopy with the use of weak beams. A procedure for determining any of possible Burgers vectors of the (1/6)〈116〉-type glide dislocations and the (1/6) 〈110〉-and (1/3)〈001〉-type sessile partial dislocations (in face-centered cubic lattices) is developed based on the criterion of the contrast value. The sessile dislocations formed by intersections of stacking faults were shown to have the (1/6)〈110〉-and (1/3)〈001〈-type Burgers vectors. The width of nonintersecting stacking faults corresponds to the stacking-fault energy ranging within 0.1–2 mJ/m².     

A theoretical interpretation of the electrical behaviour of individual edge dislocations in Si as determined by combined EBIC/TEM studies

The EBIC data obtained from individual edge and Frank partial dislocations ⁱn Si, and described in other publications, are related to defect parameters, such as the capture cross-section for minority carriers, which determine electrical recombination properties of dislocations. A two stage model for the capture of minority carriers by dislocations is developed and shown to correctly account for the temperature-dependence of the EBIC contrast obtained from these dislocations.     

Energy levels of dislocations in Ge under different conditions. Comparison between images of electron- and light-beam scanning microscopy

Thermal treatment effects on Ge deformed at 350°C are consistent with reductions of the effective dangling bond densities on the dislocations. On account of difficulties in the application of known statistics the phenomena observed are briefly discussed by considering a heavily doped cylinder model for the dislocations. Observations with infrared beam induced currents emphasize the necessity of considering the effects of other defects, introduced together with dislocations during deformation, in particular for the interpretation of photoconductivity data.