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.     

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.     

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.     

Experimental Evidence for a Step Growth Mechanism of Garnet LPE Films

Experimental observations suggesting a lateral growth mode at {110} and {211} garnet LPE films are reported. Growth steps are revealed using a gold decoration technique and a new decoration method based on the condensation of PbO nuclei is developed which allows to image the step structure of the whole film surface using light optical interference contrast. The step velocity reduced per K undercooling of spiral steps having step distances larger than 100 nm amounts to (0.70 ± 0.10) μm/min ± K independently of step density. Spiral rotation rates and critical nuclei radii are in the order of 1 s⁻¹ and 10 nm respectively.     

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.     

Vapor phase epitaxy of CdTe on sapphire substrates in dependence on the vapor-flow orientation

The growth of cadmium telluride films on a structured (0001) sapphire surface oriented at an angle of 44° to the vapor-flow direction and normal to the steps formed along the \(11\overline 2 0\) direction is studied. It is found that this geometry of the vapor source and a substrate (heated to a temperature of 300°С) provides the growth of single-crystal CdTe films if a step height on the substrate surface is more than 1 nm. The results are explained by the occurrence of a longitudinal component of the diffusion flux of СdTe molecules and atoms toward the steps from the inner side and their high density at the step edge from the outer side due to the presence of the Ehrlich–Schwoebel barrier, which ensures the efficient supply of material and minimum supersaturation necessary for the nucleation at the step edge and growth of oriented CdTe islands. The cadmium telluride films that are grown have the \(\left( {110} \right)\left[ {1\overline 1 0} \right]CdTe\left\| {\left( {0001} \right)} \right.\left[ {11\overline 2 0} \right]A{l_2}{O_3}\) orientation and a composition similar to stoichiometric CdTe.     

On the Formation of Dislocation Hollow Cores on Cleaved {100} Faces of L-Arginine Phosphate Monohydrate Single Crystals

Thermodynamic conditions and energy considerations for the formation of hollow cores at the emergence points of monolayer cleavage steps associated with screw dislocations on the cleaved {100} faces of LAP single crystals are discussed. Analysis of the formation of hollow cores and the change of the curvature of steps in the vicinity of their origins reveals that dislocations responsible for the origin of hollow cores have stress fields due to trapped mother liquor of different supersaturations. The results also show that (1) the radius of a hollow core is inversely proportional to the one-third power of the interface supersaturation while the volume of hollow corés increases exponentially with their radius, and that (2) the strain energy associated with a dislocation is responsible for the formation of an equilibrium hollow core of a particular depth at the origins of cleavage steps associated with dislocations.     

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.     

Multiatomic step formation on GaAs(001) vicinal surfaces during thermal treatment

We observed the surface morphology of vicinal GaAs(001) after thermal treatment in AsH₃/H₂ atmosphere by atomic force microscopy (AFM). Clear multiatomic steps were formed under the high temperature thermal treatment. Next, we investigated the mechanism of step bunching during thermal treatment by two experiments from the view point of Ga atom evaporation. One is the selective thermal treatment using a partially masked GaAs wafer, and the evaporation amount of Ga atoms was estimated by AFM. The other is the investigation of photoluminescence (PL) peak energy shifts for AlGaAs/GaAs single quantum wells with a thermal treatment process at the top of the GaAs quantum well layer, compared to those without thermal treatment. These results indicate that the evaporation hardly occurs during the thermal treatment process. Therefore, step bunching phenomena on GaAs(001) vicinal surfaces during thermal treatment are probably caused by migration of the atoms detached from upside steps and their re-incorporation to downside steps.     

Nature and origin of V-defects present in metalorganic vapor phase epitaxy-grown (InxAl1−x)N layers as a function of InN content,layer thickness and growth parameters

Our study of samples grown in different metalorganic chemical vapor deposition reactors and with different growth conditions reveals that V-pits are always present in (In_xAl_1?x)N films whatever the layer thickness and the InN content. V-pits are empty inverted pyramids terminating threading dislocations. InN-rich triangular regions are present around the threading dislocations terminated by pits with a hexagonal 6-fold symmetry distribution in {11?20} planes. The nature of the facets of the V-pits depends on the growth conditions: pits with either {11?2l}, l being between 1 and 3, or {1?101} facets have been observed. Moreover, the nature of the threading dislocations terminated by pits also depends on the growth conditions. Our observations suggest that with a high V/III ratio only edge a+c-type dislocations are terminated by pits whereas with a low V/III ratio both edge a-type and mixed a+c-type dislocations are terminated by pits.     

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.     

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.     

Investigations of LiInSe2 Epitaxial Layers on {100}- and {110}-oriented Substrates with Sphalerite Structure by Means of RHEED

Epitaxial LiInSe₂ films were deposited by flash evaporation technique on {100}, {110} GaAs and {100} GaP. By means of geometrical considerations based on the concept of corresponding substrate and deposit planes epitaxial relationships can be predicted and compared with results from reflection high energy electron diffraction (RHEED). The epitaxial relationships are given for both rhombic (β-NaFeO₂-type) and tetragonal (chalcopyrite-type) LiInSe₂ deposits on {100}- and {110}-oriented sphalerite-type substrates.     

Desymmetrization of the polyhedral crystal shape of tetragonal lysozyme due to face-growth rate fluctuations

We have studied the desymmetrization of the polyhedral crystalline shape of tetragonal lysozyme crystals due to the growth rate differences of the equivalent {1 0 1} planes. Using atomic force microscopy, we have observed the evolution of the multifaceted structures composed of four equivalent {1 0 1} faces during growth. In our growth condition, lateral step flow, where a large density of dislocations acts as a source of steps, is the dominant growth mechanism. The measured step flow velocities are almost independent of the separation between the neighboring steps, revealing that the local face normal growth rate is determined by the local step density. By tracing the motion of the vertex surrounded by the {1 0 1} faces, we have found that the desymmetrization of the crystalline shape is due to the large fluctuation of the local face normal growth rate, which is comparable in magnitude to the average growth rate.     

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.     

AFM study of the surface morphology of the {100} cleavage planes of L‐arginine phosphate monohydrate single crystals

Surface morphology of the {100} cleavage planes of L‐arginine phosphate monohydrate single crystals grown from aqueous solutions is described and discussed. Different from the previous studies, dislocations are not frequently detected and most of them don't have hollow cores on the emergence points. Various step patterns are also described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Atomic Force Microscopy Studies on Growth Mechanisms and Defect Formations on {110} Faces of Cadmium Mercury Thiocyanate Crystals

Growth mechanisms and defect formations on {110} faces of cadmium mercury thiocyanate crystals grown at 30°C (σ=0.24) were investigated by using atomic force microscopy (AFM). It was found that, under this condition, spiral dislocation controlled mechanism and 2D nucleation mechanism operates simultaneously and equally during growth, which is completely different from the traditional 2D nucleation and dislocation source controlled mechanisms. A number of 2D nucleus are formed at the large step terraces generated by dislocation sources, leading to the unequal growth rates of the elementary steps and thereby “step bunches” arecaused. Various defects are formed under this growth condition, which is assumed to result from the incongruence between the steps generated by different sources. A new kind of 2D defect, corresponding to one growth layer in height, was observed for the first time.     

Charge separation at evaporation and vapor growth of ice and water

A mathematical model of the interface charging at evaporation and growth of ice and water phases from vapor is proposed. This model takes into account the competition between the two mechanisms of charge separation, one of which is based on protons and the other involves orientational defects. The first mechanism leads to the accumulation of a positive charge by ice and water during evaporation, while the second one provides negative charge accumulation. The protonic mechanism dominates at low velocities of the evaporation front with respect to the condensed phase material (lower than 10^?11–10^?9 m/s). At high rates, the mechanism based on orientational defects is dominant. When vapor is condensed, and, correspondingly, the ice and water phases grow, the charge polarity is opposite to the polarity in the case of evaporation. The proposed model adequately describes the experimentally observed interface electric current and the signs of phase charges during evaporation and condensation.     

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.     

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.