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.     

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.     

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.     

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.     

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.     

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.     

Plastic deformation and fracture in LiF Bicrystals with asymmetric tilt boundary

The peculiarities of plastic deformation and fracture in isoaxial LiF bicrystals with misorientation angle α = 10° and tilt boundary of various asymmetry were experimentally studied by the single slip method. It was found that a number of effects at the boundary, attacked by mixed dislocations, is possible: retardation of dislocations near the boundary and pile-ups formation, dislocation multiplication, accumulation of grain boundary dislocations with differential Burgers vector. Predominance of each or that effect in the process of crack initiation at the definite misorientation angle depends on the mutual orientation of slip planes in the neighbouring grains and on the edge and screw component part in the dislocations, attacking the boundary. The main role, apparently, belongs to dislocation pile-ups. Cracks are always nucleated in the boundary, but in the case of strong asymmetry they stretch along cubic planes.     

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.     

Facet Formation on Near (111) Silicon Surfaces and the Desorption Energy of SiCl2 Adatoms

Silicon wafers which have a small misorientation from (111) become facetted, when they are etched beyond a critical HCl concentration. This facetting can be explained by the action of a second etch mechanism which directly attacks the surface between the steps. In this way enough new steps are generated to give a proper response to the desired etchrate. In BCF language: the spiral dissolution around a dislocation containing a screw component can become dominant over the etching process which occurs by a set of parallel steps, when beyond a critical HCl concentration the density of steps inside the shallow pit exceeds the density of the parallel steps due to the original misorientation. From the slope of the shallow pits at the critical HCl concentration a desorption energy for SiCl₂ adatoms of around 60 kcal/mole could be calculated.     

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.     

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.     

Inclined Dislocations in Synthetic Calcite Crystals

A large number of perfect (100) cleavages were obtained by cleaving the laboratory grown synthetic calcite crystals. By etching such cleavages in 20% ammonium chloride solution, it is established that the pits indicate the sites of dislocations in the crystals. This is further confirmed by comparing the etch patterns on matched cleavage surfaces. Eccentricity of the pits has been attributed to the inclined nature of dislocations. Existence of parallel dislocations seems to be due to the purity of the crystal. These observations are verified by comparing etch patterns produced on opposite faces of a thin flake (0.048 mm thick) and also by successive etching and polishing a cleavage surface. It is conjectured that curl-bottomed pits nucleate at the sites of screw dislocations in the crystal. The implications are discussed.     

Study of growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

Experimental results of the dependence of linear growth rates of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals on solution supersaturation are presented. The AO crystals were grown by constant-temperature, constant-supersaturation method at 30 and 40 °C in the supersaturation range of 1–9%. It was observed that the supersaturation dependence of growth rates follows the parabolic growth law. Analysis of the supersaturation dependence of linear growth rates of AO crystals showed (1) that growth models involving surface diffusion and direct incorporation of growth units give kinetic parameters similar to those reported for other compounds grown from solutions, and (2) that the the BCF model of cooperating screw dislocations is also applicable. An inverse relationship between the estimated values of the length, L, of the line containing the dislocations and growth rate, R, and a direct relationship between L and interplanar distance, d_hkl, of the face {hkl} were found. Both these relationships are associated with the process of generation of screw dislocations in the growing layer.     

Optical and structural properties of epitaxial GaN films grown by pulsed laser deposition

Epitaxial GaN films have been grown on c-cut sapphire substrates by pulsed laser deposition (PLD) using a KrF laser. The properties of GaN films were improved by increasing the growth temperature to 800°C and the nitrogen pressure during growth to 10 mTorr. Room-temperature photoluminescence exhibits a strong band-edge emission at 3.4 eV. From transmission electron microscopy (TEM), the predominant defects in PLD-GaN observed are stacking faults parallel to the interface and screw dislocations along c-axis, the latter differing from the previously published results where most of the threading dislocation in GaN grown by other techniques are of edge type with Burgers vector of .     

Nature and origin of V-defects present in metalorganic vapor phase epitaxy-grown (InxAl1−x)N layers as a function of InN content,layer thickness and growth parameters

Our study of samples grown in different metalorganic chemical vapor deposition reactors and with different growth conditions reveals that V-pits are always present in (In_xAl_1?x)N films whatever the layer thickness and the InN content. V-pits are empty inverted pyramids terminating threading dislocations. InN-rich triangular regions are present around the threading dislocations terminated by pits with a hexagonal 6-fold symmetry distribution in {11?20} planes. The nature of the facets of the V-pits depends on the growth conditions: pits with either {11?2l}, l being between 1 and 3, or {1?101} facets have been observed. Moreover, the nature of the threading dislocations terminated by pits also depends on the growth conditions. Our observations suggest that with a high V/III ratio only edge a+c-type dislocations are terminated by pits whereas with a low V/III ratio both edge a-type and mixed a+c-type dislocations are terminated by pits.     

Polyhedral stability ‐ on the growth of flat crystal faces on small molecule and protein crystals

The old standing problem of face morphology is discussed. A special emphasis is put on the macroscopically flat faces, appearing on small molecule crystals mostly during calm growth, under low supersaturations. As distinct, protein crystals are growing with macroscopically flat faces even under surprisingly high supersaturations. Explanations of these facts are suggested by considering the surface micro‐profile of crystal faces which growth is driven by screw dislocations. It is shown that, due to kinetic reasons, the tips of the growth hillocks and the valley between them have to be levelled to some extent (at least on a quasi‐atomic scale) under low enough supersaturations. The amplitude of the surface roughness has to be suppressed also due to the surface energy gain, especially under quasi‐equilibrium. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two‐ and three‐dimensional growth modes of nitride layers

Heteroepitaxial three dimensional (3D) and two dimensional (2D) growth modes of nitride layers on sapphire substrates are discussed. It is shown that the 3D or 2D growth mode of AlGaN layers depends predominantly on the growth conditions of the underneath low temperature (LT) nucleation layer. Commonly described in literature 3D growth mode is achieved on LT GaN or AlN nucleation layer grown relatively fast. Successive growth of secondary layer at high temperature begins from separated sites, where individual 3D crystallites are formed. Threading dislocations present in crystallites bend on their facets, which reduces the quantity of dislocations. However, slight crystallographic misorientations between crystallites lead to the creation of new dislocations during coalescence of the crystallites. As a result, edge and mix dislocations appear at similar densities of about 10⁹ cm^‐2. Modification of growth conditions of LT AlN nucleation layer, especially reduction of their growth rate, leads to drastic changes in properties of the layer. Successive growth of secondary AlGaN layer at high temperature starts evenly on whole surface retaining atomic flatness. Thus growth at high temperature occurs only by 2D mode. Therefore, it is possible to grow a very thin AlGaN layers directly on top of LT nucleation layer. Such layers contain large number (10¹⁰ cm^‐2) of edge dislocations, and relatively small number (less then 10⁸ cm^‐2) of mix dislocations. It is also shown that the decisive factor determining the growth mode of AlN nucleation layer is a growth of the first few atomic layers on substrate surface. The slow growth of these few first atomic layers decide about the 2D growth mode, and the fast one about the 3D one. The model explaining this difference is presented as well. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Deformation mechanism and dislocation structure of high-purity molybdenum single crystals at low temperatures

High purity molybdenum single crystals were deformed in tension and compression along the symmetric double slip orientation [110] in the temperature range from 300 K down to 0.5 K with strain rates between 10⁻³ and 10⁻⁵ s⁻¹. The activation volume was measured by stress relaxation tests. The dislocation structure of the deformed crystals (T → 1.85 K) was examined by high voltage electron microscopy. It was established that the low temperature increase of the critical shear stress exhibits three distinct temperature regimes with different temperature dependences. These non-uniformities are discussed in terms of recently developed theories of kink-pair formation and kink-kink interactions on screw dislocations in bcc metals. The HVEM observations suggest that the mobility of screw dislocations at very low temperatures should be determined by the combined effects of the PEIERLS barriers and the jog dragging.