Forces acting on misfit dislocations during interdiffusion

A VC doped WC-Co alloy is investigated using high resolution transmission electron microscopy. The VC grain growth inhibitor induces the presence of a thin layer on the surfaces of the WC grains in contact with Co and precipitates in the corners of Co pockets. These (VW)Cx compounds adopt an epitaxial orientation relationship with regards to the (0001) base facets of the WC crystals. Due to the small difference in lattice parameters, misfit dislocations are expected in the interfaces. Unlike the thin layers where no defects are observed, two kinds of dislocations are pointed out for larger precipitates. 1/6〈112〉VC interfacial dislocations are sometimes present while more often 1/2〈1¯10〉VC dislocations lying above the interface in the (VW)Cx phase are visible.

Behavior of misfit and threading dislocations in HgCdTe/CdTe and HgCdTe/CdZnTe heterojunctions has been investigated by the dislocation-pit etching technique. It is confirmed that by lattice matching the linear pit density caused by misfit dislocations on (1̄10) cleaved plane was decreased to 500 cm−1, which is about 1% of that obtained using CdTe substrates. It has been found that the density of the threading dislocations is not decreased by lattice matching, and nearly coincides with the dislocation density of the substrate.

Arrangement of atoms at the interface of a bicrystal system with small lattice parameter differences has been investigated through the observation of the creation of misfit or interfacial dislocations. Specimens were prepared by epitaxial deposition of pure-silicon films onto boron-doped silicon substrates. The conditions required to create misfit dislocations were investigated as a function of the film thickness for both cases of thin and thick substrates. The well-defined critical film thickness was found, and smaller values were found for the thick substrate specimens. Results are interpreted in terms of the van der Merwe theory. Effect of heat treatment upon the misfit dislocation distribution was examined. It was observed that dislocations reacted to form networks in a complicated manner. The dislocation climb rate was negligibly small compared to the diffusion length of boron impurity.

An energy-based stability analysis of misfit dislocations in two-phase electrode particles: A planar model and implications for LiFePO4

The solution of the boundary-value problem on a rectilinear screw dislocation parallel to the interface between phases with different elastic moduli and gradient coefficients is obtained in one of the versions of the gradient theory of elasticity. The stress field of the dislocation and the force of its interaction with the interface (image force) are presented in integral form. Peculiarities of the short-range interaction between the dislocation and the interface are described, which is impossible in the classical linear theory of elasticity. It is shown that neither component of the stress field has singularities on the dislocation line and remains continuous at the interface in contrast to the classical solution, which has a singularity on the dislocation line and permits a discontinuity of one of the stress components at the interface. This results in the removal of the classical singularity of the image force for the dislocation at the interface. An additional elastic image force associated with the difference in the gradient coefficients of contacting phases is also determined. It is found that this force, which has a short range and a maximum value at the interface, expels a screw dislocation into the material with a larger gradient coefficient. At the same time, new gradient solutions for the stress field and the image force coincide with the classical solutions at distances from the dislocation line and the interface, which exceed several atomic spacings.

The behavior of misfit dislocations in InP/InGaAsP/InP double-heterostructure wafers grown by liquid phase epitaxy has been investigated by x-ray and photoluminescence (PL) topography. Misfit dislocations were found to be generated from the edges of all the wafers studied here. X-ray topographs for angle-polished wafers show that misfit dislocations exist within the upper InP layer and are located in close vicinity to the heterointerface. PL topography has been used to determine the misfit dislocation-free area within the wafers. This area decreases monotonically with the increase of the upper InP layer thickness, and is almost independent of the quaternary layer thickness. The lattice misfit is necessary to be in the range 0∼−0.08% at room temperature for making the area without misfit dislocations as wide as possible. Generation of misfit dislocations is considered to be strongly related to the ’’melt back’’ of the InGaAsP edge growth.

In this study, we report that the carrier mobility of 2%-La-doped BaSnO3 (LBSO) films on (001) SrTiO3 and (001) MgO substrates strongly depends on the thickness, whereas it is unrelated to the film/substrate lattice mismatch (+5.4% for SrTiO3 and −2.3% for MgO). The films exhibited large differences in lattice parameters, lateral grain sizes (∼85 nm for SrTiO3 and ∼20 nm for MgO), surface morphologies, threading dislocation densities, and misfit dislocation densities. However, the mobility dependences on the film thickness in both cases were almost the same, saturating at ∼100 cm2 V−1 s−1, while the charge carrier densities approached the nominal carrier concentration (=[2% La3+]). Our study clearly indicates that the carrier mobility of LBSO films strongly depends on the thickness. These results would be beneficial for understanding the carrier transport properties and fruitful to further enhance the mobility of LBSO films.

Tuning of CeO2 buffer layers for coated superconductors through doping

Boundary-value problems in the isotropic theory of elasticity for a screw dislocation in the wall of a hollow cylinder and in a solid with two cylindrical voids have been solved rigorously by means of the method of virtual image dislocations. The elastic stress fields, energies and image forces on the dislocation have been analyzed in detail. In a special limiting case, the stress fields of a screw dislocation in a half-space containing a cylindrical void have been derived. The solutions obtained may be used for theoretical investigation of dislocation nucleation and behavior in hollow nanotubes as well as for a study of the formation and dynamics of cylindrical voids in plastically deformed crystals.

Data on search for correlations between a structure of state diagrams of binary systems based on V-Me (Me = Ti, Zr, Hf, Nb, Ta, Cr, Mo, W) and crystal-geometric and crystal-chemical parameters were presented. Manifestation of both positive and negative deviations in experimental concentration dependences of the atomic volume in solid solutions in systems V-Me (Me = Ti, Nb, Ta, Cr, Mo) from Zen’s law was established. It was found that in the systems with unbounded regions of solid solutions, the deviation from Zen’s law is insignificant. A correlation was found between the types of state diagrams and the difference in atomic volumes of alloy-forming elements in systems V-Me (Me = Ti, Nb, Ta, Cr, Mo).

Superconducting copper fluoride ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{1.3}{\mathrm{Cu}(\mathrm{O},\mathrm{F})}_{4+\ensuremath{\delta}}$ with ${T}_{c}=55$ K had been synthesized via the solid phase fluorination of ${\mathrm{La}}_{0.7}$${\mathrm{Sr}}_{1.3}$Cu${\mathrm{O}}_{3.25}$ with Cu${\mathrm{F}}_{2}$ at relatively low temperature. The fluoride is a derivative of its precursor, but its lattice constant a is expanded from 3.7547(5) to 3.9368(7) \AA{}, while its lattice parameter $c$ is expanded from 12.896(3) to 13.075(4) \AA{}. The obvious difference in lattice parameters and crystal structure seems to distinguish this compound from the superconducting ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{2}{\mathrm{F}}_{4+\ensuremath{\delta}}$ reported in a recent study. It gives strong evidence for superconductivity with reduced Cu${\mathrm{O}}_{2}$ sheets while in the presence of apical oxygen or fluorine.

The positions of Higher Order Laue Zone (HOLZ) Lines, visible in the bright field discs of Convergent Beam Electron Diffraction (CBED) patterns, are frequently used to determine the lattice parameters or strain present in phases observed in the Analytical Electron Microscope (AEM). The technique usually relies upon the use of a “standard” sample of known lattice parameter that has the same crystal structure as the unknown phase and is of similar average atomic number. The relative movement of particular HOLZ lines in the unknown phase with respect to the standard are used to calculate the difference in lattice parameters between the standard and the unknown. The thickness of the samples is not taken into consideration. This paper describes the results of both computer simulations and cold stage experiments in the AEM which lead us to conclude that the HOLZ lines are not independent of thickness, particularly in the presence of highly dynamical zero layer diffraction effects.

We discuss misfit dislocations (MDs) and threading dislocations (TDs) in lattice-mismatched semiconductor heteroepitaxial layers, which are the key structural elements of light-emitting diodes (LEDs) and laser diodes (LDs). Novel approaches to modelling MD formation and TD reduction are considered. The behavior of dislocations in conventional III-V semiconductor compounds as well in polar and semipolar III-nitride heterostructures are reviewed in detail.

Synchrotron x-radiation in a double-crystal geometry has been used for topography and rocking-curve measurements to study imperfections in single-crystal HP/HT synthetic diamonds grown from (111) seeds by the reconstitution technique. The influence that clusters of small opaque particles (sometimes called `pin-point-like inclusions') have upon lattice parameter differences, as well as upon relative lattice tilts within each sample, was of particular interest in this study. Rocking curves measured with a large beam diameter (illuminating entire specimens) gave widths (FWHM) in the range 5 - 30 seconds of arc in a non-dispersive Bragg - Bragg diffraction geometry in which good diamond crystals give and for which the calculated theoretical minimum width is under . Rocking curves were also recorded for small regions of each crystal, using a translatable pin-hole, and these showed that some diamonds had relatively narrow rocking curves in selected areas (of diameter). Epochs of crystal growth were recorded in series of x-ray topographs, taken at intervals of 2.5 seconds of arc. Such series were taken for all four principal azimuths (, 90, 180 and ) of the incident beam by rotating the sample about the normal to the Bragg plane. This procedure yielded important information about lattice parameter distribution and lattice tilt across entire samples. For the samples heavily populated with pin-point-like inclusions it was concluded that there is a possible slight variation in lattice parameter values in some regions of the crystal but most of it falls below the adopted angular step resolution, although better accuracy can be achieved. Relative tilt measurements revealed that there are large misorientations of those parts of the crystal which are close to external outcrops of {111} boundaries. In this case of (111) oriented diamond samples, three separate regions with different tilts can be attributed to the three parts of the crystal close to optically visible {111} growth-sector boundaries.

Small Pb inclusions in Al have been studied by a number of investigators because the alloy system offers the possibility of observing the processes of melting and solidification directly. Both solids are fee, and the mutual solubility of solid Pb and Al is negligible. Despite a large difference in lattice parameter, it has been found that inclusions follow a parallel-cube orientation relationship and their equilibrium shape is a cuboctahedron, bounded by ﹛111﹜ and ﹛100﹜ facets [1]. Following Herring, the relative extent of the two types of facet directly indicates a ratio of interfacial energies γl00/γ111- However, recent investigations have shown that for inclusions in the range of a few to a few tens of nanometers the equilibrium shape becomes a function of size [2]. In the present work, this size dependence of the equilibrium shape has been investigated further. Al alloys with about lat.% Pb were prepared by rapid solidification or by ion implantation, and equilibrated by annealing at about 300°C.