Analysis of a random system of dislocations in a deformed crystal

Structure of deformed crystals is modeled by dislocation loops of random size nonuniformly randomly distributed in natural slip planes. This model is used to determine structure parameters from the harmonics of the diffraction line of a polycrystal. Practical analysis of the dislocation structure under real conditions of the diffraction experiment is performed.     

Precipitation-induced Dislocations in Crystals of NaCl:Ni

Using the decoration technique, the dislocation structure has been observed in regions localized near precipitates in nickel-doped NaCl crystals. The results are interpreted in terms of the stress-operated mechanism for the generation of prismatic dislocations by the precipitates. Normally only two of twelve possible slip directions operate for 〈110〉 rods. This is accounted for in terms of the climb and disappearance of dislocation loops corresponding to the other directions. {100} plates produce loops spreading along for 〈110〉 directions.     

Defects in LEC-grown Indium Phosphide Crystals Doped with Tin

Crystalline defects in Sn-doped LEC indium phosphide have been revealed by chemical etching and analyzed by TEM. Grown-in dislocations, various kinds of defect clusters and colonies of microdefects were found. The symmetrical defect clusters are shown to equate mostly with larger dislocation loops exhibiting shear components and/or other dislocation arrangements generated by a stress source which is positioned in the centre of the dislocation cluster. Those centres are often formed by a plate-like agglomeration composited of tiny inclusions and very small faulted dislocation loops. Such planarly arranged accumulations of microdefects lie on {111} planes. The direct vicinity of single threading grown-in dislocations is always enriched with tiny perfect dislocation loops and precipitates. Additionally, very large isolated interstitial-type perfect dislocation loops with b = a₀/2 〈110〉 have been found by TEM experiments. Mostly, the {110} habit plane of such loops is decorated with an high number of small dislocation loops and precipitates as a consequence of dislocation climb.     

Effect of dislocation loops in macroscopically dislocation-free GaP substrates on the perfection of homo-epitactic deposits

Substrates and epitactic layers of GaP are etched under optimized conditions with the defect-revealing, preferential R-C etchant and subsequently examined using high-magnification (about 2500X) optical microscopy techniques. It is possible then to distinguish two new phenomena, viz. etch pits characteristic of dislocation loops in the substrate, and inclined dislocation dipoles in the layer. For layers grown on dislocation-free substrates we find that (i) the surface densities of both defects are equal (～5 × 10⁵cm^-2), and (ii) the average diameter of the dislocation loops in the substrate is roughly the same as the average distance between the two dislocations of the dislocation dipoles (0.5?1μm). Hence the perfection of these layers is determined by interfacial dislocation loops. Because the density of dislocation loops is only about a factor of two lower in highly (～1 × 10⁵ cm^-2) dislocated substrates, growth on these substrates results in layers which have even slightly lower dislocation density than layers grown on dislocation-free substrates. In the former case also single dislocations in the substrate propagate into the layer. Minority-carrier lifetime data indicate that minority-carrier recombination at dislocations is a restrictive factor for the luminescence quality of layers grown both on dislocation-free and highly-dislocated substrates.     

Etch-pit study of dislocation microstructure at the precipitates in NaCl:Ni crystals

The chemical etching technique was used to study the precipitation-induced dislocation microstructures in NaCl:Ni crystals with 110 rod-like precipitates. The comparison was made of the results obtained by the etch pit and decoration studies. The coincidence of the patterns suggests the prismatic dislocation loops and helices of the interstitial type to be only produced at elevated temperatures (above 300°C). The smallest size loops are removed by climb on slow cooling. These loops sometimes persist in quenched crystals.     

Structural Defects in Type I Diamonds

Natural gem diamonds of Type I single crystals were observed with an ultra high voltage transmission electron microscope in order to characterize structural defects such as dislocation, dislocation loops, and platelets. The results of analyses indicated that edge dislocations with Burgers vector 1/2 [011 1] and 60° dislocations which were inclined at an angle of 60° to 〈011〉 directions were present in the diamonds. Dislocation loops were concluded to be in interstitical character. It was speculated that an impurity associated with the platelets was silicon and dislocation loops were formed by dislocation motion.     

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.     

Modification of the crystal structure of gadolinium gallium garnet by helium ion irradiation

The structure of gadolinium gallium garnet (GGG) single crystals before and after implantation by He⁺ ions has been investigated using high-resolution X-ray diffraction methods and the generalized dynamic theory of X-ray scattering. The main types of growth defects in GGG single crystals and radiation-induced defects in the ion-implanted layer have been determined. It is established that the concentration of dislocation loops in the GGG surface layer modified by ion implantation increases and their radius decreases with an increase in the implantation dose.     

Study of Dislocation Distribution in NaCl?NaBr and NaCl?CaCl2 Crystals Grown from Melt

Dislocation distribution and content have been studied using etch pit method in the single crystals of mixed solids NaCl NaBr and NaCl CaCl₂. In both the solid solutions the dislocation content is higher as compared to that in pure crystals of NaCl and NaBr grown under similar conditions. In mixed crystals of NaCl NaBr the block structure of subgrain boundaries is not observed. The subgrain boundaries are highly fragmentary and the dislocations form arrays running randomly to short distances. In NaCl crystals doped with CaCl₂ dislocation loops and impurity-decorated dislocations are observed. The dislocation content is higher in impurity segregated regions. The impurity decorates both isolated dislocations and subgrain boundaries in calcium-doped crystals. The results are discussed.     

TEM Studies of Er+ – implanted CdTe

The structure and the depth distribution of radiation damage caused by erbium implantation (E_i = 100 keV, D = 5 × 10¹⁵ cm⁻², T_i = LNT) in 〈111〉 cadmium telluride have been investigated by means of planar-view and cross-sectional TEM techniques. It is found that the implantation disturbes the CdTe target up to the depth well-beyond the calculated projected range (that is ≳ × 5R_p). The resulting damage structure consists of well-defined defects: precipitates, stacking faults, dislocation loops, and dislocation networks, which appear to be segregated with depth forming a sequence of homogeneously faulted zones. This characteristical and deep damage is suggested to be formed on dynamic annealing in response to internal stresses (caused by lattice parameter mismatch) which are high enough to produce plastic relaxation with dislocation generation and slip. This study is aimed at describing the ion-implantation-induced damage in single crystal cadmium telluride caused by Er⁺ ions by means of transmission electron microscopy.     

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.     

TEM characterization of defects in LEC-grown GaAs substrates

The defects in semi-insulating LEC grown GaAs, perfect dislocations and polycrystalline As precipitates, are investigated by transmission electron microscopy. Vacancy type perfect dislocation loops are found to transform into incomplete stacking fault tetrahedra. In spite of the observation of arsenic precipitates, climb because of supersaturation of vacancies is found to be important for the formation of the observed dislocation configuration. This is discussed using a model of temporal succession of precipitation and dislocation climb. The most likely climb mechanism is that Ga vacancies are the driving force for a climb process during which As antisites are created.     

Effects of the Crystal Diameter on the Dislocation Density in KCl Crystals Grown by the Czochralski Method

The dependence of dislocation density on the crystal diameter of the Czochralski grown KCl crystals (both pure and Sr doped) was investigated by the method of the etch pit and X-ray topograph. (1) The dislocation density drastically decreased with the decrease in diameter (D) for small D (D < 2 mm, Fig. 2). (2) The dislocation density in the Sr doped crystal was lower than that of pure component in the diameter examined. (3) Many helical dislocations and dislocation loops were observed in the thin crystals (D ∼ 0.5 mm) of the pure KCl and Sr doped one, respectively, by X-ray projection topography. These results were discussed in connection with the dislocation generation mechanisms due to thermal stresses or supersaturated point defects.     

Etching and Dissolution Mechanism of Cadmium Oxalate Trihydrate Crystals

Dislocation etching and dissolution of gel-grown cadmium oxalate trihydrate crystals are carried out using different etchants. The study revealed the existence of dislocation network in the body of the crystal and shallow dislocation loops in the substructure. Faster dissolution of the crystal at elevated temperatures revealed that the rate of dissolution in the initial part of the dissolution time is much lesser, which increases fast to attain a steady value. This observation is related to the available specific surface area of the crystal.     

The Nature of Dislocation Damage in Electron Irradiated Al-1% Cu and Stainless Steels

Specimen of Al-1% Cu aged to contain θ′ and stainless steels of various Ni and Cr contents have been irradiated in an HVEM. It has been found that the nature of the dislocation damage in the stainless steels is a complex function of composition and temperature; very irregular faulted loops, regular faulted loops and regular unfaulted loops are formed in varying proportions. In the Al-1% Cu, aged for varying times at 190 °C to produce θ′, the ageing times influences the amount of radiation damage in a complex manner. Virtually all the visible damage in these alloys is found either at the precipitate interface or at the electron entrance surface.     

Propagation of dislocations during epitaxy

The reduction of dislocation density in epitaxial layers relative to the original density in the substrate is a well-known phenomenon which seems particularly pronounced for liquid-phase processes and in compound semiconductors. Several mechanisms have previously been suggested to account for this reduction of dislocation density. We are here proposing a simple alternative in order to explain the formation of loops and the propagation parallel to the substrate surface. The initial stages of growth are assumed to consist of advancing terraces or ridges parallel to the surface. Dislocations may be dragged along with this parallel growth, and annihilation of dislocations results. We discuss the details of this model in terms of dislocation configurations and present consequences and methods for possible experimental verifications.     

Etch- and Transmission Electron Microscope Investigations of Microdefects in (001) LEC-GaP Substrates

The etch pattern of (001) and (110) LEC-GaP doped with sulphur (N_DN_A in the range from 3–7 × 10¹⁷ cm⁻³) do not only show dislocation etch pits but fine pits, so-called “saucer pits” (S-pits). The distribution of S-pits, their etching behaviour and transmission electron microscope observations of etched samples indicated a clear correlation between S-pits and microdefects (fauted-, prismatic dislocation loops and precipitates).     

Influence of γ-Irradiation on Some Properties of NaCl:Eu++ Crystals (II). Colourability

The room temperature colouration process of NaCl:Eu++ crystals strongly depends on both the dopant concentration, represented by the absorption coefficient α(eg), and the dopant dispersion state as well. It has been found that within the UV spectral range either some V-type absorption bands appear or only the background absorption increases. For heavily irradiated crystals the clustering of halogen-interstitial results in the formation of dislocation loops. In dependence of the dopant concentration the colourability of as received crystals is either larger (α(eg) ⪅, or smaller (α(eg) ⪆ ⁻¹) of that characteristic of the solution treated ones. The difference in the effectiveness of the halogen-interstitials trapping-process has been explained in terms of the various structure and/or composition of the europium-related aggregates and/or precipitates.     

On the elements of the dislocation structure of deformed corundum

It is found by the method of transmission electron microscopy that the main elements of dislocation structure of deformed corundum are spatial triple dislocation nodes, recombination zones and dislocation network fragments. The structure of dislocation nodes is analyzed. It is shown that the nodes in corundum are the “rigid” constructions.