TEM and X-ray investigations of defect structure of PbSnTe crystal

The Pb_0.8Sn_0.2Te single crystal grown by the Bridgman technique was examined by X-ray and TEM methods. The X-ray reflection topography revealed that the PbSnTe crystal consisted of monocrystallinic blocks with linear dimensions of 1–5 mm separated by low angle boundaries of tilt-twist character. “As grown” defects observed by TEM method have been identified as single perfect dislocations and dislocation pairs. A mean density of dislocations inside the crystal blocks was no more than 10³ cm⁻². Except “as grown” defects rows of mobile dislocations were observed. All of dislocations lines were characterized by the same type of Burgers vector, i.e. b = 1/2a〈110〉.     

Plastische Verformung von Siliziumeinkristallen unterschiedlicher Ausgangsversetzungsdichte im Streckgrenzenbereich

Floating-zoned Silicon crystals orientated for single slip both dislocation-free and with grown-in dislocations (EPD₀ = 6,5 · 10⁴ cm⁻²) were dynamically deformed in compression in the temperature range 1120 … 1300 K up to the lower yield point. The dislocation structure of plastically deformed crystals (T = 1138 K; a₀ = 1,45 · 10⁻⁴ s⁻¹) was examined by etching and high-voltage electron microscopy. The crystals exhibit essential differences in deformation behaviour and in the resulting dislocation structure. Typical for the dislocation-free basic material are (i) an inhomogeneous deformation in the range of the upper yield point (ii) a greater portion of sessile lomer dislocations at the lower yield point in relation to the crystal with grown-in dislocations. The measured stresses were related to those values calculated from structure data by employing plasticity theory of diamond-like semiconductors.     

Bulk and surface components of electric influence on the magnetoplasticity

The influence of weak electric fields on the magnetically induced mobility of individual dislocations in NaCl crystals has been investigated. It is shown that the strong influence of an electric field on the magnetoplasticity, which is caused by the transformation of impurity centers at dislocations from the diamagnetic state (Me ⁺⁺) to the paramagnetic state (Me ⁺), has both bulk and surface components. The surface effect dominates over the bulk one in weak electric fields (E < 0.5 kV/m), which are insufficient for the exciting bulk electromigration of charged defects near dislocations. However, the contribution of the surface effects becomes insignificant in comparison with the bulk processes in stronger fields E. Lacquering crystal faces to block the surface electromigration of defects excludes this stimulation component at any E. It is shown that dislocation motions occurring due to a rapid switching on an electromagnet also disappear in lacquered samples. These effects observed may be related to the same surface electromigration of defects under the vortex electric fields induced by switching on a magnetic field.     

TEM-Untersuchungen von Gitterfehlern in ZnSiP2-Einkristallen

Results are dealt with concerning TEM investigations of lattice defects in ZnSiP₂ single crystals. After the crystal growth dislocations or stacking faults were found in a few cases only. More frequently twins were present in the microstructure. The crystallographic elements of twinning are {112} 〈111〉. After plastic deformation by bending at 900 °C local dislocation arrangements with high defect density (N_v ≈ 10⁶…︁ 10⁷ cm⁻²) were observed. By means of the diffraction contrast one Burgers vector b = 1/2 〈111〉 could be identified. In some cases the crystals also contained wide deformation stacking faults, which were limited by partial dislocations. The density of twins was not increased under the conditions of deformation reported here. As it can be concluded from investigations of Oettel et al. and from the results of the twin analysis, slip and generation of stacking faults take place on {112}-planes in ZnSiP₂ crystals. Crystallographic considerations on both processes are dealt with.     

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.     

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.     

Microhardness studies on as‐grown (111) faces of some alkaline earth nitrates

Single crystals of Sr(NO₃)₂, Ba(NO₃)₂ and Pb(NO₃)₂ are grown from their aqueous solutions at a constant temperature of 35 °C by slow evaporation technique. Crystals of size 8 to 10 mm along one edge are obtained in a period of 10 days. Chemical etching technique has been employed to study the dislocations in these crystals. The dislocations are randomly distributed and the dislocation density is about 10⁴ to 10⁵ /cm². Microhardness studies are made on as–grown (111) faces of these crystals upto a load of 100 g. The hardness of the crystals increases with an increase in load and thereafter it becomes independent of the applied load. These results are discussed on the basis of reverse indentation size effect. Meyer index number n for these crystals is estimated at both low and high load regions. An analysis of hardness data of these crystals as well as some other cubic crystals like alums and alkali halates are discussed using Gilman–Chin parameter H_v/C₄₄, where H_v is the microhardness and C₄₄ is the shear constant. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of the defect structure character in calcite single crystals on X-ray diffraction

The paper presents experimental results for the dependence of the integral intensity of anomalous transmission T_i and Laue reflection R_i of X-rays on the structural quality of calcite single crystals as obtained by a two-crystal spectrometer. The relation between X-ray dynamic and kinetic scattering as a function of the sample thickness, densities of perfect dislocations in perfect CaCO₃ crystals and of atom-vacancy complexes in calcite crystals with „background”︁ has been found. Distortions due to perfect grown-in dislocations in calcite are shown to exceed those produced in crystals with „background”︁. The latter appear to be more X-ray transparent. A comparison with results of twin layer thickening experiments on CaCO₃ crystals of different qualities again points to the different nature of these distortions. In CaCO₃ crystals with the dislocation density of up to 2 × × 10³ cm⁻², the X-ray scattering, in our experiments, was consistent with the dynamic diffraction theory, while at higher dislocation densities, the X-ray scattering markedly differs from it.     

Influence of Microdefects on the Generation of Dislocations in Homoepitaxial Gallium Phosphide Layers Grown on LEC Substrates

GaP LEC substrates doped with sulphur (N_D – N_A roughly (3–7) × 10¹⁷ cm⁻³) were characterized by transmission electron microscopy. This material was found to contain microdefects such as perfect perismatic dislocation loops, and spherical precipitates. Cross-sectional TEM investigations perfomed have shown that above all perfect dislocation loops lying directly at the substrate/layer interface are sources for the formation of extended dislocations propagating through the epitaxial layer. Using the methods of selective photoetching and AB-etching on (110) cleavage faces this phenomenon was observed, too.     

On the change of low-temperature thermal conductivity of gap after bending

The thermal conduction of a sulphur-doped single crystal plastically deformed by bending at 963 K was measured between 2 and 50 K. The thermal resistivity W at temperatures > 14 K is practically independent on deformation. At lower temperatures W is found to be proportional to AT⁻³ before and to AT⁻³ + BT⁻² after deformation. A is due to boundary scattering, B due to dislocations.     

Elektronenmikroskopische Untersuchungen zur Oxidkeimbildung an Reineisenoberflächen

During a high temperature treatment of pure α-iron crystals under vacuum conditions (po₂ = 1 · 10⁻⁴ … 1 · 10⁻⁶ torr) single oxide nuclei originate on the crystal surfaces. The oxide was identified as γ-Fe₂O₃. From the definite shape of the nuclei on (100) oriented α-iron and from their orientations relative to the iron lattice an epitaxial growth may be concluded, following the well-known relation (100)_Fe ∥ (100)_Oxide; [100]_Fe ∥ [110]_oxide. Grown-in dislocations, surface steps and grain boundaries of the matrix were decorated by the oxide nuclei. However, oxide nuclei did not originate at the emergent points of fresh dislocations.     

Correlation between Light Microscopical and Electron Microscopical Images of Crystal Defects in GaP

Ultramicroscopy and dark-field-illumination basing on the scattering of photons are easily available, they are nondestructive light microscopical methods for real structure investigations of semiconducting materials. The observation of dislocations is possible because of photon scattering at precipitations adherent to the dislocation line. This work gives a direct correlation between light microscopical and electron microscopical images of dislocations and in this way some hints on the nature of scattering centres. The light scattering takes place at precipitations caused due to gettering of impurity atoms by the dislocation line. Besides elongated dislocations prismatic loops were observed. The loop density was found to be in the order of magnitude of about 10¹² cm⁻³ which corresponds to a concentration of about 10¹⁷ atoms contained in the loops in good agreement with the concentration of impurities in the crystals.     

Optical defects in barium—strontium niobate single crystals

The distributions of edge dislocations and residual mechanical stresses in Ba_xSr_1-xNb₂O₆ (BSN) crystals are investigated and the explanation of the nature of the “growth column” is proposed. The “growth column” is a defect zone going through all of the crystal and usually repeating in its cross-section the contour of the seed crystal. The “growth column” boundary is the closed contour with extremely high edge dislocation density. These dislocations are connected with thermal stresses due to seed-melt contact or abrupt crystal widening. Under proper crystal seeding and widening conditions one can obtain the BSN crystals with dislocation densities less than 10 cm⁻² and without the “growth column”. The method of chemico-mechanical polishing of BSN crystals not forming a defect layer on the surface of the crystals have been developed. The high temperature diffusion annealing is shown to eliminate the growth striae in BSN crystals.     

Efficiency of dislocations as sinks of radiation defects in fcc copper crystal

The sink efficiency of perfect dislocations for self-point defects (interstitials and vacancies) in fcc copper crystal has been calculated by the kinetic Monte Carlo method in a temperature range of 293–1000 K and a range of dislocation densities from 1.3 × 10¹² to 3.0 × 10¹⁴ m^?2. Screw, mixed, and edge dislocations with a Burgers vector 1/2<110> in different slip systems are analyzed. The interaction energies of self-point defects with dislocations are calculated using the anisotropic theory of elasticity. Analytical expressions are proposed for the dependences of the calculated values of dislocation sink efficiency on temperature and dislocation density.     

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)     

Determination of the types and densities of dislocations in GaN epitaxial layers of different thicknesses by optical and atomic force microscopy

The change in the dislocation density on the surface of GaN epitaxial layers, which were grown by hydride vapor-phase epitaxy on sapphire substrates with c and r orientations, has been investigated by optical and atomic force microscopy (AFM). It is shown that the observed decrease in the density of threading dislocations with an increase in the layer thickness is related to the annihilation of mixed dislocations. The experimental and theoretical data on the change in the density of mixed dislocations with an increase in the epitaxial-layer thickness are in good correspondence.     

Nucleation of Cracks in CdS during Ultrasonic Treatment

High amplitude ultrasonic vibrations with frequencies f_r ∼ 90 kHz and vibration amplitudes ε ⩽ 3.4. 10⁻⁴ were used to obtain oscillatory stresses σ ⩽ 14 MPa in CdS single crystals. Such treatment leads to the nucleation of cracks in annealed samples and to the formation of dislocations in as-grown samples. The different behaviour can be explained by the reduction of the free dislocation segment lengths due to the diffusion of point defects towards the dislocation lines during the annealing treatment.     

Effect of buffer layers on the dislocation structure of heteroepitaxial systems

Transmission electron microscopy (TEM) as well as X-ray topography (XRT) and X-ray diffractometry have been used for investigation of the structure of the LPE heteroepitaxial system In_0.05Ga_0.95As-In_yGa_1−yAs_1−xP_x-GaAs(111) A. A critical value of the lattice misfit has been shown to exist at the metallurgical boundary ((Δa/a)* ≈ 10⁻³) which results in the change of the film nucleation and growth mechanism as well as the change of misfit dislocations (MDs) generation mechanism. With (Δa/a)₀ > (Δa/a)* the nucleation and growth mechanism is mixed: island growth at the first stages of growth and layer-by-layer growth at large thicknesses. MDs are created in an “island film” developing a non-ordered dislocation network. The density of threading dislocations (N_d) is ∼ 10⁸ cm⁻². With (Δa/a)₀ < (Δa/a)* there is layer-by-layer mechanism of film's nucleation and growth from the very first stages of crystallization. MDs are injected into continuous layer along the inclined slip planes {111}, thus forming a regular three-dimensional grid of MDs. N_d is less than 10⁶ cm⁻² in the case. A model of dislocation structure formation in heterolayers has been proposed. Within the frame of this model the two critical values of phosphorus concentration in the quaternary melt have been quantitatively determined. These are corresponding to the change of MD generation mechanism. The expected values of N_d for (Δa/a)₀ > (Δa/a)* and (Δa/a)₀ < (Δa/a)* have been theoretically determined.     

Optical birefringence images of dislocations in large gallium phosphide crystals

The optical birefringence induced by the strain field of a dislocation has been used to follow dislocation lines through large gallium phosphide crystals. Dislocations could be seen whether their lines were parallel to the imaging light beam or were steeply inclined to it. The images obtained so far suggest that it would be possible to resolve individual dislocations in crystals that contain ?5 × 10⁵ dislocations per cm².     

Dislocations in magnetic garnet crystals

1. Dislocations in magnetic flux-grown garnet crystals (Y₃Fe₅O₁₂ and others) have been observed. As a rule, {110} growth pyramids have more defects than {211} ones. 2. The highest content of defects (dislocation density 10³–10⁴ cm⁻²) is observed in Y₃Fe₅O₁₂ that grows on crucible walls adjacent to the free surface of the solution where the flow of heat is not uniform to the greatest degree. Bottom grown crystals usually have less dislocations. Far fewer dislocations are in wall grown crystals, least of all dislocations are contained in crystals that grow inside the solution. The solution pouring off at the end of the crystallization period increases dislocation density by some dozens. 3. Heat treatment decreases dislocation density. The less dislocation content and the lower ordering are in the initial crystal, the higher heat treatment effectiveness.