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.     

On the Formation of Dislocation Hollow Cores on Cleaved {100} Faces of L-Arginine Phosphate Monohydrate Single Crystals

Thermodynamic conditions and energy considerations for the formation of hollow cores at the emergence points of monolayer cleavage steps associated with screw dislocations on the cleaved {100} faces of LAP single crystals are discussed. Analysis of the formation of hollow cores and the change of the curvature of steps in the vicinity of their origins reveals that dislocations responsible for the origin of hollow cores have stress fields due to trapped mother liquor of different supersaturations. The results also show that (1) the radius of a hollow core is inversely proportional to the one-third power of the interface supersaturation while the volume of hollow corés increases exponentially with their radius, and that (2) the strain energy associated with a dislocation is responsible for the formation of an equilibrium hollow core of a particular depth at the origins of cleavage steps associated with dislocations.     

Growth mechanisms,polytypism, and real structure of kaolinite microcrystals

The mechanisms of growth of kaolinite microcrystals (0.1–5.0 μm in size) at deposits related to the cluvial weathering crust, as well as to the low-temperature and medium-temperature hydrothermal processes of transformations of minerals in different rocks in Russia, Kazakhstan, Ukraine, Czechia, Vietnam, India, Cuba, and Madagascar, are investigated using transmission electron microscopy and vacuum decoration with gold. It is established that kaolinite microcrystals grow according to two mechanisms: the mechanism of periodic formation of two-dimensional nuclei and the mechanism of spiral growth. The spiral growth of kaolinite microcrystals is dominant and occurs on steps of screw dislocations that differ in sign and magnitude of the Burgers vector along the c axis. The layered growth of kaolinite originates from a widespread source in the form of a step between polar (+ and ?) dislocations, i.e., a growth analogue of the Frank-Read dislocation source. The density of growth screw dislocations varies over a wide range and can be as high as ～10⁹ cm^?2. Layered stepped kaolinite growth pyramids for all mechanisms of growth on the (001) face of kaolinite exhibit the main features of the triclinic 1Tc and real structures of this mineral.     

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.     

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.     

Plastic Deformation of BCC Metals — Open Problems

For a deeper understanding of the plastic behaviour of bcc metals a detailed knowledge of the mobility of screw dislocations, which is intimately connected with their core structure, is necessary. A critical review of our present knowledge indicates an almost absolute lack of quantitative information on screw dislocation mobility at finite temperatures. Therefore, a detailed experimental and theoretical study of these processes is an open problem of great importance. Such a study would be of great value for the entire field of plastic deformation of bcc metals.     

Nature of dislocations promoting growth in liquid phase epitaxy of gallium arsenide

Dislocations promoting growth in the course of liquid phase epitaxy (LPE) of GaAs layers on GaAs substrates are analysed by X-ray topography. The Burgers vectors are determined by comparing double-crystal back-reflection images with calculated misorientations taking into account surface relaxation. Any dislocation which generates a spiral of elementary steps is found to have a Burgers vector component parallel to the macroscopic growth direction. The nature of these growth promoting dislocations may be between pure screw and pure edge type. Defects which might be responsible for the generation of the observed concentric growth step patterns are below the detection limit of current X-ray topography.     

Anomalously Large Burgers Vectors of Screw Dislocations in Gallium Nitride Nanowires

Crystallography Reports - The results of structural studies of GaN nanowires containing screw dislocations are presented. It is found that the length of the Burgers vector of dislocations may reach...     

Crystallisation Mechanism in Gradient Temperature Zone Melting

Theory of zone recrystallization in the framwork of already known models of normal crystal growth, growth by screw dislocations and by two-dimensional nucleation is discussed. By mathematical treatment, different from Tillers approach, analytical expressions for supersaturation at the crystallisation interface, for superheating at the dissolution interface, growth rate and some other parameters have been obtained for both cases of normal and screw dislocations growth. It is possible to determine the growth mechanism.     

Evaporation Model of NaCl Cleavage Faces around Edge Dislocations in Vacuum

It is shown in the present paper that the evaporation of edge dislocations in the form of circular steps takes place in NaCl crystals by the evaporation of these steps in a high vacuum and in a temperature region of 350–500°C. The distances of steps are regular. This regularity is explained by means of a suggested model. This model explains also the change in the evaporation rate depending upon the density of dislocations. This explanation is based on the existence of an effective charge at the emergence point of the edge dislocations a/2 〈110〉. {110} with the surface planes {100}. This charge interacts with another one formed on the faults of steps. As a result of this interaction a modulation is observed of the formation of a new step in the emergence point. This means that the same is observed in the case of step distances 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.     

Method of Evaluating Geometrical-Statistical Parameters of the Interaction between Dislocations and Point Obstacles from Electron Micrographs

Electron micrographs of MgO crystals deformed in the HVEM show screw dislocations of curly shape. The dislocations are pinned at localized obstacles and bow out between them. The paper describes a measuring procedure for the geometrical interaction parameters between the dislocations and the obstacles. Such are the effective stress, the obstacle distance and the force acting on the obstacles. The procedure consists in a graphical fit of loops calculated by the DE WITT-KOEHLER line tension model to the observed dislocation segments.     

Observation of individual dislocations in 6H and 4H SiC by means of back‐reflection methods of X‐ray diffraction topography

SiC crystals of high structural perfection were investigated with several methods of X‐ray diffraction topography in Bragg‐case geometry. The methods included section and projection synchrotron white beam topography and monochromatic beam topography. The investigated 6H and 4H samples contained in large regions dislocations of density not exceeding 10³ cm^‐2. Most of them cannot be interpreted as hollow core dislocations (micro‐ or nano‐pipes). The concentration of the latter was lower than 10² cm^‐2. The present investigation confirmed the possibility of revealing dislocations with all used methods. The quality of presently obtained Bragg‐case multi‐crystal and section images of dislocation enabled analysis based on comparison with numerically simulated images. The analysis confirmed the domination of screw‐type dislocations in the investigated crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanism of spatial ordering of dislocations during plastic deformation of crystals

Mechanisms of spatial ordering of dislocations during plastic deformation of crystals are considered. The system of evolution equations, which take into account the effects of elastic and correlated interactions of screw dislocations, is derived. The study is performed with due regard for the dynamics of spatial fluctuations of the dislocation density.     

Transmission electron microscopy observations of twin boundaries and sub-boundary networks in bulk CdZnTe crystals

The microstructure characteristics of the twin boundaries and sub-boundary networks in bulk cadmium zinc telluride (CdZnTe) crystals have been studied by transmission electron microscopy (TEM). Three types of twin boundaries were identified and characterized, which are (i) single straight twin boundary, (ii) tilt twin boundary, and (iii) twin boundary with steps. Boundary dislocations at tilt twin boundary and high dislocation density around steps were observed. The origin of the boundary dislocations is ascribed to the lattice misfit between two tilt grains. The formation of the twin boundary with steps is suggested to be the interaction between the twin boundary and dislocations. Honeycomb-like sub-boundary defects were also observed. The probable reason for the formation of the sub-boundary networks is discussed.     

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.     

A model for the growth of anomalous polytype structures in vapour grown SiC

A number of polytype structures observed in vapour grown SiC crystals have a unit-cell which is an integral multiple of the unit-cell of the basic 6H, 15R or 4H structure. The growth of such anomalous structures cannot be understood in terms of spiral growth round a single screw dislocation in a basic matrix. However many of these polytype crystals display a single growth spiral on their (0001) face indicating that they have resulted from spiral growth round a single screw dislocation. It is shown that this anomaly can be resolved if the basic matrix is assumed to contain stacking faults near the surface at the time of the origin of the screw dislocation ledge. This possibility, overlooked in the earlier deduction of polytype structures, must be taken into consideration since vapour grown SiC crystals frequently contain a high concentration of random stacking faults, producing continuous streaks on their X-ray diffraction photographs. The most probable fault configurations that can occur in 6H, 15R and 4H structures of SiC have been deduced from a calculation of their stacking fault energy. These fault configurations are then considered to lie at different distances from the surface at the time of the origin of a screw dislocation ledge. Such a faulted ledge gives rise to polytype structures during subsequent spiral growth even if the screw dislocation has an integral Burgers vector. The most probable series of polytype structures that can result from such a faulted matrix model are deduced. It is shown that nearly all the polytype structures of SiC hitherto regarded as anomalous (such as 36H, 54H, 66H, 45R, 90R etc.) are among the expected structures and there is no need to postulate a complicated configuration of cooperating dislocations to account for their growth.     

X-ray and scanning cathodoluminescence imaging of small-angle grain boundaries and dislocations in CdTe crystals

By employing the Reflection X-ray Topography (RXT), on one hand, and the Cathodoluminescence (CL-IR) mode of operation of the Scanning Electron Microscope, on the other hand, wer are able to visualize the lattice defects in CdTe single crystals. The possibility of defect identification is the advantage of the RXT technique. Moreover, the examined surface can be imaged as a whole. Cathodoluminescence, however, guarantees higher resolution. The defects in CdTe crystals, observed in cathodoluminescence images, have been identified as small-angle boundaries and screw dislocations.     

Lattice plane imaging in ZTA ceramics

Characteristic interface phenomena have been studied in zirconia toughened alumina (ZTA) using high-resolution electron microscopy. Glassy phases in grain triple junctions, microstructure of interfaces between martensitic twins, regularly spaced misfit dislocations at the interface between intragranular t-ZrO₂ and the Al₂O₃ matrix and amorphization processes in the twin-terminating region (which can initiate grain-boundary microcracking) were observed by lattice plane imaging techniques.