Defect Structure of Strained Heteroepitaxial In(1—x)Al(x)P Layers Deposited by MOVPE on (001) GaAs Substrates

Weak-beam, large angle convergent beam electron diffraction and high resolution transmission electron microscope experiments have revealed, that after strain relaxation due to plastic deformation dislocation networks can be observed in In_(1—x)Al_(x)P heteroepitaxial layers grown on (001) GaAs substrates under compressive stress. The 60° slip dislocations are mostly dissociated into partials of Shockley type whereas in the particular case of layers grown under tension twins are predominantly formed by successive nucleation and slip of 90° Shockley partials on adjacent {111} glide planes lying inclined to the (001) surface. When a few 90° Shockley partials pile up during extension of twins, then planar incoherent twin boundaries with {112} coincidence planes have been formed during strain relaxation. Due to the space group symmetry ((InAl)P belongs to the space group F4-3m) there is a striking asymmetry in defect formation, i.e. defect nucleation and slip on the planes (111) and (1-1-1) slip of the [1-10] zone are preferred to nucleation and slip on the {111} planes of the [110] zone. Apparently, the occupacy of the atomic sites in the dislocation core with either group-III or group-V atoms is responsible for this behaviour. The nature of the defects implies that their spontaneous nucleation should have taken place at the growing surface. Under tensile strain the 90° Shockley partial is nucleated first and the 30° one trails. Under compressive strain this sequence is reversed. It is evident, for dissociated dislocations lying at the interface always the 30° partial, i.e. the partial with less mobility or with higher friction force, is detained near or directly in the interface. Thus, in layers grown under tension the stacking fault associated with the dissociated 60° dislocation lies inside the GaAs substrate. For layers grown under compression it is located inside the ternary layer.     

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.     

Dynamics of twin layer broadening in indium

The mobility of plane-parallel twin layer boundaries in indium crystals has been studied in the 235–390 K temperature range under stresses, τ, of (0.25–20) g/mm² (static load) and (35–335) g/mm² (crest values under impact load). For τ/G = (0.2–0.6) × 10⁻⁵ (G is the shear modulus), the process of twin layer broadening has been shown to be thermally activated; the process parameters and their stress dependence have been found. – Possible mechanisms of twin layer broadening have been analysed, and it has been concluded that a pole mechanism is inadequate to treat the results obtained. The twin broadening is considered to be due to nucleation and motion of twinning dislocations along boundaries in each subsequent twinning plane; it has been shown that the thermal activation parameters measured while broadening a twin layer, can be inconsistent with the elementary acts of broadening process (nucleation or motion of twinning dislocations). A deep gap between data on stress dependence of the activation energy of twin layer broadening for indium and calcite crystals and the Sumino theory is explained by the determining influence of the real imperfect structure of specimens on the process studied.     

Transmission electron microscopy observations of twin boundaries and sub-boundary networks in bulk CdZnTe crystals

The microstructure characteristics of the twin boundaries and sub-boundary networks in bulk cadmium zinc telluride (CdZnTe) crystals have been studied by transmission electron microscopy (TEM). Three types of twin boundaries were identified and characterized, which are (i) single straight twin boundary, (ii) tilt twin boundary, and (iii) twin boundary with steps. Boundary dislocations at tilt twin boundary and high dislocation density around steps were observed. The origin of the boundary dislocations is ascribed to the lattice misfit between two tilt grains. The formation of the twin boundary with steps is suggested to be the interaction between the twin boundary and dislocations. Honeycomb-like sub-boundary defects were also observed. The probable reason for the formation of the sub-boundary networks is discussed.     

A crowdion mechanism of crystal twinning

The processes of plastic deformation of crystals via mechanical twinning are considered. Models of nucleation and motion of twin boundaries leading to formation of twin interlayers are proposed. These models are based on the crowdion mechanism of deformation in crystals and are considered by an example of close-packed structures. Good agreement between the predictions of the models proposed and the experimental results for crystals deformed by a concentrated load is demonstrated.     

Defect Structure in Te-doped GaAs single Crystals after Plastic Deformation (I). Twins and Stacking Faults

At temperatures above the brittle-to-ductile transition (490 °C) in Te-doped GaAs three types of predominant defect configurations have been observed after uniaxial compression along a [001] direction: (i) twins and stacking faults (500 … 520 °C), (ii) slip zones of dislocations (≈ 550 °C) and (iii) dislocation cells (580 … 590 °C). In Part I quantitative details of the appearance of twins and stacking faults are given. Most frequently found were 30° partials in twins and stacking faults.     

Defect Investigations in CdTe Crystals by RXT,SCL, and EBIC(V) Techniques

The paper covers results of investigations on the defect structure of CdTe crystals. Several techniques have been involved, namely RXT supplied with CL-IR, EBIC(V), and SACP using Electron Probe Micro-Analyzer (EPMA): EBIC(V) measurements were performed on Schottky diodes with CdTe-Au contacts. It has to be emphasized that microscopic images at small magnification (45 ×) had successfully been obtained which were next correlated with X-ray topographs. The observed due to grain boundaries, twins, “as grown” dislocations as well as those introduced either during high-temperature plastic deformation or by pyramidal diamond indenter are discussed.     

Growth twins in self-frequency doubling laser crystal YbxY1−xAl3(BO3)4

Using chemical etching.method, the growth twins in self-frequency doubling laser crystal Yb_xY_1−xAl₃(BO₃₄ have been observed. The etching pits on both sides of growth twin boundaries in the (10 1) slice are of the triangles with different orientations. The structure of growth twins is investigated by transmission synchrotron topography. In the transmission synchrotron topograph, the growth twins are visible not by ‘domain contrast’ but by ‘boundary contrast’, i.e. the twins appear in the topograph in form of X-ray kinematical diffraction contrast due to the lattice strain stemming from the impurity incorporation in the boundaries. The growth twins in Yb_xY_1−xAl₃(BO₃)₄ crystal are of inversion types, since no domain contrast was observed.     

Studies on twin boundaries in calcite crystals

Natural single crystals of calcite have been cleaved along (100) planes and cleavage faces have been etched in 2% and 3% citric acid solutions. Etching produces twin boundaries oriented in 〈010〉 directions. The etch pits on the two sides of the twin boundary are oppositely oriented. It has been conjectured that the rows of pits might have been formed due to etching of dislocations on twin boundaries. One to one correspondence of twin boundaries has been established on matched cleavage faces. This is further confirmed by studying the induced twin regions produced on a (100) cleavage plane by indenting that plane itself. The implications are discussed.     

On microcrack nucleation at twin vertices and boundaries in bcc and fcc metals

Nucleation of microcracks at the vertex and boundary of a decelerated twin is studied for a number of bcc and fcc metals. A twin and its boundary are represented as stepped pileups of twinning dislocations located in the neighboring glide planes. The formation of microcracks through the merging of head dislocations by the force and thermally-activated mechanisms is analyzed. Analytical expressions are obtained that describe the conditions necessary for microcrack nucleation at the vertices of stepped dislocation pileups. It is established that with an increase in the shear-modulus value, the critical parameters of microcrack nucleation by the two above mechanisms become closer in all the metals considered.     

Crystal structure of stacking faults in InGaAs/InAlAs/InAs heterostructures

Stacking faults and dislocations in InGaAs/InAlAs/InAs heterostructures have been studied by electron microscopy. The use of different techniques of transmission electron microscopy (primarily, highresolution dark-field scanning transmission electron microscopy) has made it possible to determine the defect structure at the atomic level.     

Twins in CdMnTe single crystals grown by Bridgman method

Cd_1‐xMn_xTe (x =0.2, CdMnTe) crystal was grown by the vertical Bridgman method, which exhibits a pure zincblende structure in the whole ingot. The major defect, twins, which is fatal to CdMnTe crystal, was analyzed with scanning electron microscopy (SEM), X‐ray energy disperse spectroscopy (XEDS) and optic microscopy on the chemical etching surface. The twins observed in the as‐grown ingot are mainly lamellar ones, which lie on the {111} faces from the first‐to‐freeze region of the ingot and run parallel to the growth axis of the ingot. Coherent twins with {115}_t‐{111}_h orientations when indexed with respect to both the twin and host orientations, are often found to be terminated by {110}_t‐{114}_h lateral twins. Te inclusions with about 20 μm in width are observed to preferentially decorate the lamellar twin boundaries. The origin of the twins, relating to the growth twin and the phase transformation twin, is also discussed in this paper. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bildung von α-Martensit im Schnittbereich von ϵ-Martensitplatten

In steels the nucleation of crystals of α martensite from austenite is facilitated in the presence of ϵ martensite. Particularly within the range of crossing platelets of ϵ martensite with 〈110〉_γ direction α martensite is formed easily. That has been demonstrated by metallography in alloys with suitable composition and pretreatment. Also within the range of crossing of other shear products, as stacking faults or twins, the atomic arrangement for nuclei of α martensite is favourable. By the aid of matrices the strain in crossing 2 ϵ martensite plates, ϵ martensite plate and twin, and 2 twins resp., has been calculated and the result compared with the strain during the martensitic γ → α transformation.     

Influence of impurity atmosphere on the deformation of silicon crystals

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.     

Defect generation on mechanically surface defects before and after the oxidation (II). Dislocation generation at cracks

The generation of crystal defects at residual cracks due to a wet oxidation of chemically mechanically polished silicon wafers at 1100 °C was investigated. It was found that the nucleation of oxidation stacking faults at these elastic cracks is impeded as compared to the regions between the cracks. However, extended dislocations, dislocation dipoles, certain stacking faults, and precipitates are generated at such cracks. These crystal defects and the crack itself impede the nucleation of oxidation stacking faults at the crack, as they locally reduce the excess interstitials at the oxidation front.     

Etching Figures in Neodymium Gallate and Yttrium Aluminate Crystals

The dislocations existing in single crystals of neodymium gallate and yttrium aluminate grown by the Czochralski technique have been studied by means of etch pits. The data concerning their solubility in cases of different directions of a face orientation, various treatment temperatures and several enchant types are reported. The investigation of etch pits in the twinned YAlO₃ and NdGaO₃ crystals showed that twins are formed during a growth process. In the [110]‐pulled NdGaO₃ crystals the discrepancy between the twin and matrix parts of a crystal is accommodated by the dislocation congestion and the dislocation low‐angle boundaries whereas in [010]‐pulled YAlO₃ crystals the microcracks perform this function.     

Dynamic Aspects of Dislocation Behaviour Near Low-Angle Boundary in Molybdenum Single Crystals

The dislocation behaviour was inventigated in specimens of monocrystals of molybdenium during in — situ stretching in the direction [001] near low — angle twist boundary which was close to be parallel to the direction of the external force. Low-angle boundary is easily penetrable for the fast moving nonscrew dislocations and acts as an effective stopper for screw dislocations which are quite parallel to the forming boundary dislocations. The mixed tipe dislocations emission by the low-angle boundary was noticed. On the stage when plastic deformation is performed mainly by the motion of screw dislocations the “relay-race”-like transmission of the dislocations motion through the boundary was observed.     

Crystallography of the Martensitic transformation in Indium-Thallium Alloys

On cooling a single crystal of Indium containing 20.75 atomic per cent Thallium may transform martensitically by the migration of a single interface, covering the whole cross-section, from the high temperature face-centred cubic β-structure to the facecentred tetragonal α-structure. Applied external mechanical stress is able to reverse the direction of interface motion during transformation. The α-martensite consists of a stack of fine parallel twins which are suficiently wide to be visible under an optical microscope. There are no longrange stresses, provided the twins have mean thickness ratio 1.6:1. By extension or compression the whole specimen may undergo a conversion into one or the other orientation by movement of the twin boundaries, leading to ferroplasticity or ferroelasticity. The geometry and crystallography of and the effects of external stresses on the β → α transformation have been calculated on the basis of the phenomenological crystallographic martensite theory.     

Defect generation on mechanical surface defects before and after the oxidation (I). TEM investigations on the nucleation of oxidation stacking faults at mechanically induced weak surface defects

In order to obtain information on the nucleation mechanism of oxidation stacking faults, mechanically induced weak surface defects as simulated by low-load scratches were investigated with TEM before and after a short-time wet oxidation at 1050 °C. The experimental results could not confirm the existing models which explain stacking fault nucleation by the splitting of complete dislocations. Rather it was found that clusters of silicon interstitials are generated due to strong local compression during scratching, which then act as nuclei for stacking faults in the subsequent oxydation process.     

X-Ray topographic characterization of α-Fe2O3 single crystals obtained by chemical transport with TeCl4

The method of Lang is used to study the defect structure of „as-grown”︁ hematite crystals obtained by chemical transport with TeCl₄. The presence of twin boundaries, dislocations and crystallites built in the platelet matrices is established. Considerable differences in density and distribution of the crystal disturbances are observed with samples grown under the same experimental conditions. In some cases the real structure is found to be nearly perfect.