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.     

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.     

HVEM In-situ Deformation Investigations on Cyclically Deformed Nickel Single Crystal

In-situ deformation investigations were carried out on thin foils of a nickel single crystal which was cyclically pre-deformed. The motion of dislocations does not start in the whole specimen simultaneously but begins at localized places in the dislocation-poor regions. The plastic deformation may be characterized by a collective movement of dislocations. Measurements of the effective stress show its characteristic dependence on the position in the direction of the Burgers vector. There are clear indications of structural changes in the dense regions of dislocations at strains exceeding the cyclic total strain amplitude.     

沿[1-100]方向升华法生长6H-SiC单晶

本文采用升华法沿着垂直于c轴方向的[1-100]方向生长6H-SiC单晶.利用光学显微镜对晶体表面及腐蚀后的晶片进行观察,发现沿[1-100]方向生长出的单晶与传统方法沿[0001]方向生长单晶有很多的不同之处,多型对于籽晶的继承性非常强,但是在生长过程中多型夹杂不会发生,该方法生长的晶体中没有发现螺位错(微管)缺陷.     

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.     

Defect characterization and composition distributions of mercury indium telluride single crystals

A mercury indium telluride (MIT) ingot was grown by the vertical Bridgman method. The defects in MIT crystals were characterized by the chemical etching method. A defect etchant for MIT crystals was developed. The etch pits of dislocations, microcracks and boundary was observed by scanning electron microscopy. It was elucidated that the etch pits density of dislocations of MIT wafers was about 4×10⁵ cm⁻². Te and In reduced at the grain boundaries, but were homogeneously distributed within the grains in the as-grown MIT crystals. The distribution of In in MIT crystals along the growth direction and radial direction was analyzed by electronic probe microscopy. It was found that In concentration was higher in the initial part and lower in the final part of the MIT ingot, which indicated that the segregation coefficient of In in MIT crystals was 1.15. The radial In concentration increased from the center to edge of the wafers and homogeneous in the middle part.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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...     

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.     

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 .     

Computerberechnete Atomanordnungen in elektronenmikroskopisch beobachteten facettenförmigen Phasengrenzflächen im gerichtet erstarrten Al?Al3?Ni-Eutektikum

The directionally solidified Al Al₃Ni eutectic consists of an Al-matrix and of facetted Al₃Ni-rods grown into it nearly parallel to each other and to the direction of heat flow. — The orientations of the interfacial planes observed by electron microscopy were determined relative to both crystal lattices. The atomic arrangement in crystallographic planes of both phases parallel to these observed interfaces has been computed and drawn by a special program. The mating projections of their atomic packing were compared together in order to estimate the lattice misfit, atomic matching and atomic densities of the boundary layers. While the most favoured habit plane (001)Al₃Ni is explainable in terms of attaining low interfacial free energy through low misfit, similar atomic arrangements and densities, the higher indexed planes could not be verified by an appropriate concept due to their complex, incoherent nature and similar size.     

Low-angle boundary investigation in silicon single crystals

Low-angle boundaries in Si single crystals grown from the melt in [111], [100], [112], [110], [118] and [115] directions were investigated by chemical etching, copper decoration technique and X-ray topography. — LAB of four types were found in planes parallel to the crystal growth axes. Rearrangement of dislocations from slip bands into LAB was observed in heavily-doped Si crystals. The origin of LAB in melt grown Si crystals is discussed. It is shown that these boundaries are well interpreted in terms of dislocation alignment formation in the thermal stress field of the growing crystal.