Calculation of displacement fields and simulation of HRTEM images of dislocations in sphalerite type A(III)B(V) compound semiconductors

The displacement fields of different kinds of both perfect and dissociated dislocations have been calculated for an isotropic continuum, and by means of linear elasticity. Additionally, the corresponding HRTEM images have been simulated by the well-established EMS program package in order to predetermine the structural aspects of dislocations, and then to compare it with experimental HRTEM micrographs. The latter ones resulted from plastically deformed GaP single crystals and InAs/(001)GaAs single epitaxial layers. It could be established that using the simple approach of linear elasticity and isotropy results can be obtained which correspond well to the experimental images. So, the structure of various Shockley partial dislocations bounding a stacking fault can be detected unambiguously. The splitting behaviour of perfect 30° dislocations (separation into a 0° and 60° partial) and 90° dislocations (separation into two 60° partials) both with line direction along 〈112〉, 60° dislocations (separation into 30°/90° and 90°/30° configuration) and screw dislocations (separation into two 30° partials) along 〈110〉 are discussed in the more detail. Moreover, the undissociated sessile Lomer dislocation, glissile 60° dislocation and edge dislocation have been considered too.     

Defect Structure in Te-doped GaAs Single Crystals after Plastic Deformation (II). Dislocation Slip and Cell Formation

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 II quantitative details of the appearance of slip and cell formation are given. Leading segments of gliding half loops are mainly of 60° type. Cell walls were formed by multiple slip of perfect 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.     

Stability of dislocations in MgO single crystals at high temperatures

The dislocation structures of MgO single crystals annealed at 1900 °C for 3 hrs and at 2000°C for 1 hr were observed electron microscopically, and it was found that the following reactions took place at high temperatures: a/2 〈11 0〉 + a/2 [110], = a [100], a/2 [110] + a/2 〈11 01〉 = a/2 [011]. The resultant dislocations were sessile. Their interaction with impurities would make the subgrain boundaries stable.     

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.     

Edge misfit dislocations in the GeSi/Si(001) pair: Conditions and specific features of high-quantity generation

Dislocation structure of Ge_xSi_1?x films (x=0.4?0.8) grown by molecular-beam epitaxy on Si(001) substrates was studied by means of transmission electron microscopy. It was found that the density of edge MDs formed at the early stage of plastic strain relaxation in the films could exceed the density of 60° MDs. In our previous publications, a predominant mechanism underlying the early formation of edge misfit dislocations (MD) in Ge_xSi_1?x/Si films with x>0.4 was identified; this mechanism involves the following processes. A 60° glissile MD provokes nucleation of a complementary 60° MD gliding on a mirror-like tilted plane (111). A new edge MD forms as a result of interaction of the two complementary 60° MDs, and the length of the newly formed edge MD can then be increased following the motion of the “arms” of the complementary 60° MDs. Based on this scenario of the edge MD generation process, we have calculated the critical thickness of insertion of an edge MD into GeSi layers of different compositions using the force balance model. The obtained values were found to be more than twice lower than the similar values for 60° MDs. This result suggests that a promising strategy towards obtaining dislocation arrays dominated by 90° dislocations in MBE-grown Ge_xSi_1?x/Si films can be implemented through preliminary growth on the substrate of a thin, slightly relaxed buffer layer with 60° MDs present in this layer. The dislocated buffer layer, acting as a source of threading dislocations, promotes the strain relaxation in the main growing film through nucleation of edge MDs in the film/buffer interface. It was shown that in the presence of threading dislocations penetrating from the relaxed buffer into the film nucleation of edge MDs in the stressed film can be initiated even if the film thickness remains small in comparison with the critical thickness for insertion of 60° MDs. Examples of such unusual MD generation processes are found in the literature.     

Defects in natural diamonds: Recent observations by new methods

The observations presented concern both grown-in defects and those which develop after growth. They will deal principally with impurity platelets precipitated on {100} planes and with dislocations. The former defects are a notable feature in the most common variety of natural diamond (Type Ia.) The experimental techniques used are of the direct-imaging, topographic type and include ultraviolet absorption topography, X-ray topography, cathodoluminescence topography (both with visible and near infrared wavelengths) and transmission electron microscopy at 100 kV, 120 kV and 1 MeV. Spectrography and colour photography are informative in cathodoluminescence studies. The cathodolumisnecent images of individual dislocation lines can be photographed, whether the dislocations be grown-in or arise from plastic deformation post growth, provided that they lie in matrices sufficiently free from point defects acting as strong electron-hole recombination centres. In practice this requirement implies a low concentration of nitrogen impurity much of which is distributed in “small clusters” which give rise to the bright blue cathodoluminescent emission characteristic of Type Ia diamonds. The luminescence from individual dislocation lines may be either a violet-coloured emission, or an emission which is dominantly a system (known as “H3”) having a zero-phonon line at 2.46 eV and stretching from green to orange in the visible spectrum. The violet dislocation emission is strongly polarised with E vector parallel to the dislocation line. New observations on platelets have been mainly concerned with those attaining diameters of 1 μm or more which can be observed individually by X-ray topography and cathodoluminescence topography. However, evidence from electron and X-ray diffraction contrast indicates that these “giant platelets” produce the same matrix lattice displacements as the more familiar submicrometre-sized platelets in Type Ia diamond and may be presumed to have essentially the same structure and composition. Near infrared cathodoluminescence has been recorded from individual large platelets and it has been discovered that this emission is highly polarised with the E vector in the platelet plane. Available evidence on the structure and properties of the platelets weighs against a currently mooted hypothesis that they are composed of interstitial carbon, and does not conflict with an older idea, that they are composed of nitrogen impurity.     

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.     

Characterization of defects in GaP and GaAsP graded heterojunctions by transmission electron microscopy

Misfit dislocations are observed in graded heterojunctions GaAs_1?xP_x by electron microscopy. Results are in agreement with previous work concerning the nature of the dislocations (Lomer and 60° dislocations) and their density dependence on the phosphorus gradient. The discussion concerns the formation of Lomer dislocations and the possibility of reducing the density of inclined dislocations which reach the surface of the epitaxial layer. GaP substrates, S-doped, are examined by transmission electron microscopy. Numerous defects such as Frank loops, perfect loops, helical dislocations and precipitates are observed. A GaP homoepitaxial layer realized on this substrate is free from these defects but exhibits stacking faults. A zinc diffusion does not produce additional defect but a 1000 Å thick amorphous layer is observed a at the surface.     

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.     

Nucleation,Glide Velocity and Blocking of Misfit Dislocations in SiGe/Si

Relaxation of strained layer systems is still not well understood. It is time dependent and changes considerably for samples with different growth history. This has to be discussed in terms of nucleation, glide velocity and blocking of misfit dislocations. We have investigated these phenomena at samples with SiGe layer thicknesses ranging from 60 nm up to 120 nm grown by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) by means of X-ray topography. The samples were annealed at temperatures between 500° and 600° C. Nucleation of misfit dislocations is heterogeneous and the rate is obviously dependent on layer strain and thickness. A quantified nucleation rate was not yet accessible, mainly due to the preferential formation of dislocation bundles. The propagation velocities of misfit dislocation segments were measured during annealing by means of synchrotron radiation plane wave topography (reflection geometry). The values agree well with theory and there is no evidence that they depend on growth regime. It is shown that the interaction of propagating misfit dislocations with crossing ones may lead to blocking or cross slip in different glide systems. These results are corroborated by investigations with atomic force and transmission electron microscopy.     

Dislocations and dislocation reduction in space grown GaSb

The behaviour of dislocations in GaSb crystals grown in space both from a stoichiometric melt (floating zone method, FZ) and a Bi solution (floating solution zone, FSZ) respectively, is studied. Predominantly straight 60° dislocations with Burgers vectors of the type b = a/2 <110> in (111) glide planes are identified. In the 20 mm long FZ single crystal the linear growing out of the dislocations is observed which reduces the dislocation density in the centre of the crystal to values below 300 cm^–2. The Bi incorporation in the FSZ crystal results in a misfit between seed and grown crystal and in a network of misfit dislocations at the interface. Thermocapillary convection during growth as well as the surface tension may be the reasons for the presence of curved dislocations and the higher dislocation density within a 1 – 2 mm border region at the edges of both of the crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.     

Dislocation densities in copper single crystals after thermal treatment

Copper crystals of high purity and doped with nickel were subjected to a long time thermal treatment at 1050°C, in order to reduce the as-grown dislocation density of about 10⁷ cm⁻². A dissolution of the dislocations during their motion to the crystal surface was observed. Regions of different dislocation densities arose. Finally the crystals exhibited constant dislocation densities of 10² cm⁻². Such a perfection or a better one of the crystals is reached only by a long time treatment, since the dislocation velocity without external stresses cannot exceed 1 · 10⁻⁷ cm · s⁻¹ in copper single crystals of high purity.     

Realstrukturuntersuchungen von hydrothermal auf kleinen Rhomboederflächen gewachsenen Quarzeinkristallen

Crystals of α-quartz, which were grown from seeds of the minor rhombohedral face (1 011) and the basal face (0001), were examined with optical, etching and x-ray topographic methods. In agreement with the literature we have observed that there is a heavier coloration due to Al-impurity after x-ray irradiation, in the (1 011)-growth direction, than in others; the basal growth sector remains colourless. The strong coloration must not be in agreement with a high dislocation density. The dislocations in the rhombohedral growth are characteristically different from that of the basal growth. Correlation of the observed surface structure, as hillocks, terraces, and spirals with the distribution of etch pits is not observable.     

Plastic deformation and fracture in LiF Bicrystals with asymmetric tilt boundary

The peculiarities of plastic deformation and fracture in isoaxial LiF bicrystals with misorientation angle α = 10° and tilt boundary of various asymmetry were experimentally studied by the single slip method. It was found that a number of effects at the boundary, attacked by mixed dislocations, is possible: retardation of dislocations near the boundary and pile-ups formation, dislocation multiplication, accumulation of grain boundary dislocations with differential Burgers vector. Predominance of each or that effect in the process of crack initiation at the definite misorientation angle depends on the mutual orientation of slip planes in the neighbouring grains and on the edge and screw component part in the dislocations, attacking the boundary. The main role, apparently, belongs to dislocation pile-ups. Cracks are always nucleated in the boundary, but in the case of strong asymmetry they stretch along cubic planes.     

Specific features of the formation of dislocation structure in gallium arsenide single crystals obtained by the Czochralski method

The influence of the deviation of seed orientation from the [100] direction on the formation of a dislocation structure of gallium arsenide single crystals grown by the Czochralski method has been revealed. The intensive multiplication of dislocations and formation of a block structure occur at deviation by an angle of more than 3° in the region that is radially shifted to one of crystal sides. The linear density of dislocations in the walls changes from 1 × 10⁴ cm^–1 in low-angle boundaries to 6 × 10⁴ cm^–1 in subboundaries.     

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.     

Growth of InGaAs/GaAs on Offcut Substrates by MOVPE: Influence on Macrosteps and Dislocations Formation

A study of the relationship between the macrosteps caused by the substrate misorientation and dislocation nucleation in MOVPE-grown InGaAs/GaAs is presented. The macrosteps could favour strain relaxation and the decrease of the critical thickness, also by generation of misfit dislocations in the 1/2〈110〉{011} glide system, as they can provide sites for stress accumulation above the average value far from the macrosteps. This adds up to the enhanced homogeneous dislocation nucleation associated with the offcut angle. The use of offcut substrates thus produces both compositional inhomogeneities and an increase of the overall dislocation density.