Pairs Of Stacking Faults Research Articles

Twin nucleation from disconnection-dense sites between stacking fault pairs in a random defect network

Deformation twinning, an important plastic deformation mode, critically influences the strength and ductility of Mg. As such, a fundamental understanding of twin nucleation will provide us with an important insight into the conditions that favor the onset of this deformation mode. To reveal the twin nucleation mechanism, we construct a defect network without making assumptions about the types and relative densities of dislocations present. The generated defect network features a high density of I1 stacking faults as well as disconnections between them. Deformation modeling results suggest that this structure enables a twin nucleation event from disconnection-dense sites between I1 stacking faults pairs. By probing twin nucleation and early-stage growth inside of the random defect network, we propose a geometry-based twin variant selection rule as well as a pure-shuffle twin nucleation and early-stage growth mechanism underpinning twinning in Mg.

Activation of a hybrid twinning mechanism in a Cr-Ni-Si-V-N medium manganese austenitic steel containing precipitates

Twinning nucleation in a medium Mn austenitic steel containing precipitates is studied at low strain ~ 0.02, using transmission electron microscopy. In the near vicinity of the precipitates, unfaulting dislocation reaction is favored, while twinning is activated farther from the precipitates. Twin nucleation follows a hybrid mechanism, involving creation of stacking faults through classical dislocation dissociation, while those stacking faults subsequently overlapped following a non-classical alternated stacking fault pair mechanism to create a three-layer twin nucleus. The observed twinning behavior is interpreted from an energy barrier variation viewpoint within the matrix.

Microscopic model for the stacking-fault potential and the exciton wave function in GaAs

Two-dimensional stacking fault defects embedded in a bulk crystal can provide a homogeneous trapping potential for carriers and excitons. Here we utilize state-of-the-art structural imaging coupled with density functional and effective-mass theory to build a microscopic model of the stacking-fault exciton. The diamagnetic shift and exciton dipole moment at different magnetic fields are calculated and compared with the experimental photoluminescence of excitons bound to a single stacking fault in GaAs. The model is used to further provide insight into the properties of excitons bound to the double-well potential formed by stacking fault pairs. This microscopic exciton model can be used as an input into models which include exciton-exciton interactions to determine the excitonic phases accessible in this system.

On the activation of alternated stacking fault pair twinning mechanism in a very large-grained Fe–29Mn–2.4Al steel

An alternated stacking fault (SF) pair twinning mechanism involving local fcc → hcp transformation in a very large-grained high-Mn austenitic steel is reported, which hitherto was valid for nano crystalline metals. It was identified using high-resolution transmission electron microscopy that a pre-twinned ε-martensite like phase, containing local hcp structure is converted to a stable three-layer twin lamella through nucleation of new SF in-between the pre-existing SFs. Activation of observed mechanism is deciphered in terms of the low intrinsic and unstable stacking fault energies ratio of the steel (~ 0.13), predicted using a model based on density functional theory.

High Quality 3C-SiC Substrate for MOSFET Fabrication

Quantitative efficacies of several methods for stacking fault (SF) reduction are evaluated using Monte Carlo (MC) simulation. SF density on a 3C–SiC {001} surface depends on interactions of adjoining SFs: annihilation between counter pairs of SFs and termination by orthogonal SF pairs. However, SFs are not entirely eliminated when growth occurs on undulant-Si and switch back epitaxy (SBE) due to spontaneous SF collimation that suppresses the annihilation probability of counter SFs. The MC simulation also reveals the efficacy of SF reduction method which includes the growth of 3C–SiC on finite area bounded by side walls. One can theoretically reduce the SF density below 100 cm-1 on 3C–SiC {001} surface. A practical way for eliminating the SF by termination at side walls is demonstrated, and it clearly exhibits that the SF density can be reduced under 120 cm-1.

Control of the stacking fault areas in pseudomorphic ZnSe layers by photo-molecular beam epitaxy

Pseudomorphic ZnSe layers on GaAs(0 0 1) were grown by molecular beam epitaxy under the light illumination with photon energy of about 1.8 eV. In the layers, isolated Shockley-type stacking faults on (1 1 1), bordering not on the ZnSe/GaAs interface but on the ZnSe surface, as well as the well-known stacking fault pairs, were formed. The sum of the stacking fault areas was small in comparison with the layers grown by molecular beam epitaxy without light illumination.

Anisotropic distribution of stacking faults in (Ga,Mn)As digital ferromagnetic heterostructures grown by low-temperature molecular-beam epitaxy

We report on the microstructure of (Ga,Mn)As-based digital ferromagnetic heterostructures, which nominally consist of 40 periods of 0.75-monolayer (ML) Mn sheets between 17-ML GaAs spacer layers grown on GaAs(001) substrates by low-temperature molecular-beam epitaxy. Transmission electron microscopy studies reveal mainly stacking faults, which are preferentially coupled in V-shaped pairs with short intersecting lines along the [11¯0] direction. With increasing the V/III beam equivalent pressure ratio, a stronger laterally inhomogeneous distribution of the Mn atoms is detected along the sheets resulting in a larger local strain and thus in a higher density of stacking fault pairs. Their anisotropic distribution is explained by the energetically favorable Mn–As bonding configuration that is induced by the specific surface morphology appearing at the low growth temperature.

Transformation of Shockley into Frank stacking faults in a ZnS 0.04 Se 0.96 /GaAs (001) heterostructure

We report the transformation of Shockley partial dislocations (PDs) into Frank PDs in lattice-matched ZnS 0.04 Se 0.96 /GaAs(001) as investigated by transmission electron microscopy. The ZnS 0.04 Se 0.96 layers, with a nominal thickness of 70 nm, were grown on GaAs(001) by metal-organic chemical vapour deposition at 350°C. We mainly find stacking-fault pairs on the (111) and planes that are bound by Shockley PDs with Burgers vector . Different reactions are observed between PDs taking place in situ in the electron microscope, leading to the transformation of Shockley PDs into Frank PDs with and stacking faults on the or planes.

Stacking-fault-induced pairs of localizing centers in ZnSe quantum wells

Pairs of bright spots are observed in microphotoluminescence intensity maps of ZnSe/ZnMgSSe quantum-well structures. These pairs are exactly aligned parallel to the [1 1 0] or [ 1 ̄ 1 0] directions. Atomic-force microscopy and plan-view transmission electron microscopy reveal that the bright emission spots are related to pairs of stacking faults. The enhanced radiative recombination is a result of exciton localization in the region where the stacking faults intersect the quantum wells. Cross-section transmission electron microscopy and microphotoluminescence spectroscopy show that in the case of Frank-type stacking faults (oriented along [1 1 0]) the wells are enlarged by up to 12 bilayers. The exciton localization is much shallower in the case of Shockley-type stacking-fault pairs (oriented along [ 1 ̄ 1 0] ).

Radiative recombination centers induced by stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures

Stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures are found to induce enhanced radiative recombination visible as pairs of bright spots in microphotoluminescence intensity maps. Structural investigation by atomic-force microscopy and transmission electron microscopy (plan view as well as cross section) reveal that a widening and bending of quantum wells occurs when they are intersected by Frank-type stacking faults. The enlargement of the well width by up to 12 bilayers evokes an efficient localization of excitons. The localizing potential related to Shockley-type stacking-fault pairs is found to be much shallower.

Weak-beam analysis of dissociated 1/2«112] superdislocations in gamma-TiAl

Among the problems encountered in the characterization of 1/2<112] super dislocations in gamma-TiAl intermetallic alloy, one arises from asymmetric or abnormal contrast observed during transmission electron microscopy (TEM) experiments. The diffraction contrast from the core of these superdislocations does not always follow the rules of diffraction contrast established for analyzing partial dislocations bounding stacking faults. In particular, the configuration involving three similar Shockley partials on adjacent planes has often been ruled out owing to the absence of fringes indicating the presence of stacking faults. In order to determine the dissociated configuration, weak-beam TEM observations of edge-oriented 1/2<112]superdislocations have been correlated with computer-simulated images. Dissociation of these superdislocations into similar 1/6<112] Shockley partial dislocations bounding a superlattice extrinsic and intrinsic stacking-fault pair has been consequently determined from these analyses. Comparison of experimental and simulated images confirms that the formation of an antiphase boundary or complex stacking-fault-linked dissociation or locking by stair rod dislocations can be ruled out.

Analysis of Weak-Beam Contrast from SESF/SISF Fault Pairs Associated with ½ &lt;112] Superdislocations in TiAl

ABSTRACTThe diffraction contrast from dissociated ½&lt;112] superdislocations in γ-TiAl intermetallic alloy cannot always be analyzed using conventional rules of diffraction contrast. In particular, the configuration involving three similar Shockley partials on adjacent planes has often been ruled out due to the absence of fringes indicating the presence of stacking faults. In order to determine the dissociated configuration, weak-beam transmission electron microscope observations of edge-oriented ½&lt;112] superdislocations have been correlated with computer simulated images. Dissociation of these superdislocations into three similar ⅙&lt;112] partial dislocations bounding a superlattice extrinsic and intrinsic stacking fault pair has been consequently determined from these analyses. It has been found that diffraction contrast alone cannot distinguish between the various configurations that lead to the formation of the fault pair, but the formation of an antiphase boundary or complex stacking fault linked dissociation or locking by stair rod dislocations can be ruled out.

Local interface composition and extended defect density in ZnSe/GaAs(001) and ZnSe/In0.04Ga0.96As(001) heterojunctions

We have recently shown that in II–VI/III–V heterojunctions and related devices fabricated by molecular beam epitaxy, the II/VI flux ratio employed during the early stages of II–VI growth can be used to control the local interface composition and the band alignment. Here we demonstrate that the local interface composition in pseudomorphic, strained ZnSe/GaAs(001) heterostructures as well as lattice-matched ZnSe/In0.04Ga0.96As(001) heterostructures also have a dramatic effect on the nucleation of native stacking fault defects. Such extended defects have been associated with the early degradation of blue-green lasers. We found, in particular, that Se-rich interfaces consistently exhibited a density of Shockley stacking fault pairs below our detection limit and three to four orders of magnitude lower than those encountered at interfaces fabricated in Zn-rich conditions.

Stacking faults in pseudomorphic ZnSe-GaAs and latticematched ZnSe-In0.04 Ga0.96 As layers

ABSTRACT We report transmission electron microscopy studies of native extended defects in pseudomorphic ZnSe/GaAs (001) and lattice-matched ZnSe-In0.04 Ga0.96 As (001) heterostructures. The dominant defects present in the layers were identified as Shockley stacking fault pairs lying on (111) and (111) fault planes and single Frank stacking faults lying on (111) or (111) fault planes by comparing experimental images with the predictions obtained with the g b = 0 rule as well as with simulated images.

Interface composition and stacking fault density in II-VI/III-V heterostructures

The Zn/Se flux ratio employed during the early stages of molecular beam epitaxy of pseudomorphic ZnSe/GaAs(001) as well as lattice-matched ZnSe/In0.04Ga0.96As(001) heterostructures controls the density of the native stacking faults, which have been associated with the early degradation of blue-green lasers. In particular, the density of Shockley stacking fault pairs decreases by three to four orders of magnitude and that of Frank stacking faults by one order of magnitude in going from Zn-rich to Se-rich interfaces.

An atomistic study of dislocations and their mobility in a model DO 22 alloy

Since basic features of the plastic behavior of intermetallic compounds are often related to the structure of dislocation cores, we investigate here the cores of [1 1 0] and ( 1 2 )[11 2] screw dislocations in a model D0 22 alloy. First the stability of planar faults on {111} and {001} planes is analyzed and possible dislocation dissociations discussed. The only glissile dislocation with a planar core is the Shockley partial 1 6 [11 2] bounding a superlattice intrinsic stacking fault; however, the other superpartial bounding the same fault, 1 3 [11 2] , is sessile. Under the applied stress an intrinsic-extrinsic stacking fault pair is generated at the latter partial and this configuration may then act as a nucleus for mechanical twinning. Such fault pairs have been observed in Ni 3V. Another deformation mode in the model alloy is the slip of 〈110〉-type dislocations but since these dislocations are sessile, this requires thermal activation. The major deformation modes of Ni 3V and Al 3Ti are, indeed, 〈112〉 twinning and, at high temperatures, 〈001〉{111} slip.

Dissociation of 1/2 &lt; 112 &gt; perfect dislocations and nature of slip dislocations in ordered Ni3V

Electron microscopy observations show that in the ordered alloy Ni 3V, perfect 1/ 2[112] dislocations are dissociated into three 1/ 6[112] partial dislocations bounding an intrinsic-extrinsic pure stacking fault pair (i.e. without any wrong first neighbour bonds); from this an evaluation of stacking fault energies can be deduced. Furthermore, such pairs should extend under stress and produce 1/ 6[112] partials as the slip dislocations in this crystal structure.

Investigations on ‘doping stacking fault’ pyramids

So-called ‘doping stacking fault’ pyramids discovered in Sb-doped Czochralski pulled silicon single crystals have been shown to be invisible in the electron microscope and in X-ray topography. Several hypotheses were examined in order to find an explanation for the nature of the faults. The idea that planes of foreign atoms (Sb) might be built into the crystal, or that a single-plane stacking fault (with R = − 1 4 [111] ) might be present had to be abandoned. The assumption of intrinsic/extrinsic stacking fault pairs composing the pyramids appears to be most likely. The possibility of the occurance of doping stacking faults together with ‘normal’ stacking fault pyramids in epitaxial wafers is discussed.