The role of structural defects in diffusion of elements of group VI in semi-insulating gallium arsenide crystals

The combined effect of the changes in the number and type of vacancies and dislocation density on selenium and sulfur diffusion in single crystals of undoped semi-insulating gallium arsenide has been studied. The differences in the diffusion mechanisms in the subsurface region of samples with an initial deficiency in gallium or arsenic are established as well as the dependence of the effective radius of arsenic trapping by dislocations on the ratio of the concentrations of gallium and arsenic vacancies.     

Horizontal bridgman growth of gaas single crystals

In order to meet requirements for the preparation of low dislocation density GaAs single crystals a horizontal Bridgman type apparatus was designed. Construction of the apparatus allows the establishment of enhanced temperature stability of the arsenic source and hence better stability of the arsenic vapour pressure during the growth. This effect was achieved without application of heat pipes which are usually used for the improvement of thermal profile and temperature stability. The influence of different arsenic vapour pressures on the density of dislocations was confirmed in the course of this work.     

Influence of non-stoichiometry on the recombination activity of dislocations in undoped semi-insulating GaAs crystals

The effect of non-stoichiometry of gallium arsenide melt composition on the recombination activity of dislocations in undoped semi-insulating GaAs crystals is studied and analyzed. It is shown that the deviation from the stoichiometric melt composition is one of the major factors affecting the recombination activity of dislocations in undoped semi-insulating GaAs crystals. namely: the recombination activity of dislocations is increased when the arsenic-rich melt is used, and, on the contrary, is decreased when the gallium-rich melt is used. The regularities observed are explained by the complex processes of interaction of dislocations with the non-stoichiometry-induced intrinsic point defects.     

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.     

Microplasticity of CsI crystals

The study of the mechanisms of plastic deformation of CsI crystals has found the participation of not only the main {110} 〈100〉 slip system but also of the secondary one {110} 〈110〉. Besides that, production and motion of point defects (or small prismatic loops) take place. The gliding on secondary slip system and the deformation accounted for by the generation and motion of point defects is facilitated at low temperatures and high deformation rates. The character of the motion and multiplication of dislocations in the main slip system is investigated. From the temperature and stress dependence of the mobility of isolated dislocations quantitative data on the thermally activated motion of edge dislocations on the main slip system have been obtained. It is shown that, as in the case of other alkali halides, the thermally activated motion of edge dislocations in CsI crystals on {110} 〈100〉 system is limited by their interaction with local obstacles.     

Optimization of the Growth Technology for Crystals with Garnet Structure Based on X-Ray Diffraction Data

High efficiency of the methods of double-crystal X-ray diffractometry (DCXRD) and topography for improving the growth technology of highly homogeneous crystals has been demonstrated on the example of gadolinium gallium garnet (GGG) single crystals. The main types of structural defects observed in Czochralski-grown GGG crystals are found to be macroscopic inhomogeneity of composition distribution, caused by the facet effect manifestation; microinhomogeneous distribution of impurity and main components of the composition in striations; dislocations; and second-phase inclusions. The relationship between the type and density of newly formed defects and the technological conditions for crystal growth are considered. Optimization of the composition of crystals and their growth technology made it possible to obtain high-quality dislocation-free crystals of GGG and complex-substituted garnets on its basis for magneto-optical and microwave devices, elements of solid-state lasers, and other applications.     

Effect of thermal annealing conditions on the microdefect formation in undoped GaAs single crystals grown by Czochralski method

The effect of thermal annealing conditions on the microdefect formation in undoped GaAs single crystals grown by the Czochralski method has been studied by X-ray diffraction and metallographic analyses. It is found that the standard heat treatments performed with the aim to relieve elastic stresses and to increase the homogeneity of wafers substantially affect microdefects formed in the crystal. Upon annealing, the micro-defects in ingots and wafers exhibit different behavior. Prolonged annealing leads to an increase in the sizes of large microdefects but does not suppress the formation of small-sized microdefects. The latter defects are formed at T< 950°C upon cooling from the annealing temperature, and their number strongly depends on the density of dislocations, which serve as sinks for intrinsic point defects.     

On the delay of thermally activated motion of pyramidal dislocations in zinc

The effect of shear stress and temperature on the delay time, t_i, of thermally activated motion of dislocations in the {112 2} 〈112 3〉 slip system in Zn single crystals is studied. The delay is associated with a higher density of crystal defects at the initial path of dislocation motion.     

Growth of defect-free Sr1−xBaxNb2O6 (SBN) crystals

SBN single crystals are grown by the Czochralski technique. Growth defects (striations, dislocations, twin lamellae) are investigated as depending on growth conditions. Crystals almost free from growth defects and applicable to optical devices are prepared by an optimized growth technology.     

X-ray and scanning cathodoluminescence imaging of small-angle grain boundaries and dislocations in CdTe crystals

By employing the Reflection X-ray Topography (RXT), on one hand, and the Cathodoluminescence (CL-IR) mode of operation of the Scanning Electron Microscope, on the other hand, wer are able to visualize the lattice defects in CdTe single crystals. The possibility of defect identification is the advantage of the RXT technique. Moreover, the examined surface can be imaged as a whole. Cathodoluminescence, however, guarantees higher resolution. The defects in CdTe crystals, observed in cathodoluminescence images, have been identified as small-angle boundaries and screw dislocations.     

Analysis of synthetic diamond single crystals by X-ray topography and double-crystal diffractometry

Structural features of diamond single crystals synthesized under high pressure and homoepitaxial films grown by chemical vapor deposition (CVD) have been analyzed by double-crystal X-ray diffractometry and topography. The conditions of a diffraction analysis of diamond crystals using Ge monochromators have been optimized. The main structural defects (dislocations, stacking faults, growth striations, second-phase inclusions, etc.) formed during crystal growth have been revealed. The nitrogen concentration in high-pressure/high-temperature (HPHT) diamond substrates is estimated based on X-ray diffraction data. The formation of dislocation bundles at the film-substrate interface in the epitaxial structures has been revealed by plane-wave topography; these dislocations are likely due to the relaxation of elastic macroscopic stresses caused by the lattice mismatch between the substrate and film. The critical thicknesses of plastic relaxation onset in CVD diamond films are calculated. The experimental techniques for studying the real diamond structure in optimizing crystal-growth technology are proven to be highly efficient.     

Thermochemical analysis of native point defects and substitutional carbon and boron in GaAs crystal growth

The phase extent of GaAs has been analyzed and compared with published phase diagrams as related to total and partial point defect equilibria including charged and uncharged Frenkel, Schottky, antisite defects and substitutional carbon and boron on both sublattices. The well-known transition between semiconducting and semi-insulating behaviour at 300 K as a function of melt stoichiometry in LEC crystals can be reproduced in our model in which complete equilibrium exists above, only electronic equilibrium below a freeze-in temperature of 1100 K. The corresponding model standard enthalpy of formation of neutral Schottky defects is 4.0 eV, of a pair of neutral uncorrelated antisite defects 3.8 eV, of neutral Ga Frenkel defects 4.1 eV and of neutral As Frenkel defects 3.6 eV. Defect reactions in cooling processes after crystal growth are discussed and shown to be quite different for crystals with high or low dislocation density. Semi-insulating behaviour requires the existence of carbon acceptors if dislocations provide internal sources and sinks for point defects. For ideal crystals carbon would not be necessary. The possible site distribution of C and B is analyzed in its dependence on temperature and chemical potential of As. Constitutional supercooling is negligible in LEC growth. Macrosegregation is severe if the As fraction in the melt deviates more than ±0.02 from the stoichiometric value 0.5.     

Anisotropy of the optical suppression of the magnetoplastic effect in NaCl single crystals

Anisotropy of the plane-polarized light effect on magnetosensitive point defects in NaCl crystals has been observed. The data obtained may yield the information on the symmetry of the electronic states of these defects.     

Transmissionselektronenmikroskopische Untersuchung der Defektstruktur der intermetallischen Verbindung MgZn2

Lattice defects in single crystals of MgZn₂ (hexagonal) have been investigated by electron transmission microscopy. In both not-deformed and deformed single phase samples dislocations have been observed. In crystals with an excess of magnesium partially coherent precipitates of the adjacent phase Mg₂Zn₃ appeared.     

Melt nonstoichiometry and defect structure of ZnGeP<Subscript>2</Subscript> crystals

The defect structure of ZnGeP₂ crystals grown from a melt by the vertical Bridgman method has been investigated. A deviation of the melt composition from stoichiometric leads to the formation of striations and the inclusions of other phases which are observed as structures (chains) oriented parallel to the growth axis. According to the microanalysis data, the inclusion composition corresponds to a mixture of ZnGeP₂, Zn₃P₂, and Ge. Nanoinclusions of germanium phosphide are detected by transmission electron microscopy. X-ray topography reveals defects of four types. The main defects in the central part of an ingot are related to the composition fluctuations, and the newly formed dislocations are basically single ones. Most dislocations are formed at the crystal periphery.     

Analysis of X‐ray extinction in crystals with inhomogeniously distributed dislocations

The secondary extinction theory of Zachariasen for mosaic crystals as well as the formalism of Becker&Coppens have been used for substructure analysis (lattice disorientations, block size, density of excess dislocations, etc.) in crystals with inhomogeniously distributed dislocations in the Bragg case of diffraction geometry. In the case of large crystals D_S ≫ Λ_hkl the mean total density ρ_D of randomly distributed dislocations was also estimated taking into account additionally the primary X‐ ray extinction treatment. In this connection two cases are considered: I) pure secondary extinction related to arrangement of dislocation walls and II) mixed extinction in crystals with large subgrains related to randomly distributed dislocations and arrangement of dislocation walls. In order to check the considerations, the experimental and calculated data were compared for Be (model I) and Cu (model II) single crystals. The weakly distorted single crystals of Be and Cu were experimentally investigated in the Bragg case of diffraction geometry using Cu Kα₁ radiation by means of double crystal diffractometer. A new experimental procedure was proposed. Using alternative technique for substructure analysis (for instance EBSD) the reliability of analysis based on extinction phenomenon in weakly distorted single crystals has been checked. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on the Growth of Triglycinsulphate Single Crystals

Crystallization of TGS at 52.0°C - above the transition point - has been studied in a wide range of supersaturation of the solution (σ = 0 to 10⁻²). The rates of growth of {110} and {001} faces were measured as a function of supersaturation at constant hydrodynamical conditions (Re = 3.4 · 10⁻³). Further, the influence of hydrodynamical conditions on the growth of {110} faces at constant supersaturation (σ = 4.2 · 10⁻³) was established. The parameters of the experimentally found dependences are determined on the basis of the surface-diffusion model of BURTON . CABRERA and FRANK . From these dependences follows that the growth rate of the {110} faces is already almost limited by the volume diffusion of TGS molecules towards the crystal surface, while in the case of {001} faces the surface diffusion mechanism of crystallization is clearly manifested. Dislocation densities in the crystals have been determined by means of etching technique. The number of dislocations increases with increasing supersaturation; hence, supersaturation of the solution together with the processes taking place in the regeneration zone surrounding the seed determine the number of dislocations in the crystal volume and thus the resulting structural perfection of single crystals. Investigation of the spontaneous redistribution of domains showed that the growth rate of TGS crystals influences the dielectric properties to much smaller extent than does chemical purity.     

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.     

Heterometry-induced defects and stresses in isomorphously mixed crystals of varying composition

The comparative study of the distribution and density of dislocations and cracks in isomorphously mixed potassium-rubidium biphthalate crystals grown under the conditions of discrete and continuous changes of the solution composition with time. The method is developed for calculating the heterometry-induced internal stresses and the character of their variation in a crystal during its growth. It is established that formation of dislocations, cracks, and interzonal inclusions is associated with the effect of the heterometry-induced stresses, whereas the number of defects depends on the values of these stresses varying during crystal growth.