Distribution Of Misfit Dislocations Research Articles

Atomistic approach to the strain field in finite-sized heterostructures

The numerical code AS_LBFG_v2 developed to simulate the atomic elastic relaxation in finite-sized heterostructures is introduced. To minimize the elastic energy the L-BFGS algorithm is utilized. A user-defined initial atomic configuration, without restrictions on the shape of the matrix embedding quantum dot (QD), can be implemented in the code. By enlarging the pyramidal QD heterostructure towards mesoscopic size, the code generates a strain field distribution similar to that obtained by continuum model for weakly polar semiconductors. The strain field obtained for semi-torus, and spherical shape QDs embedded in finite-sized matrices is also presented. The dependence of the strain field on the cap thickness and the distribution of misfit dislocations in the relaxed configuration are simulated by the code. Program summaryProgram titles:AS_LBFG_v2, AS_Calc_v2CPC Library link to program files:https://doi.org/10.17632/53dshkskct.1Code Ocean capsule:https://codeocean.com/capsule/5342364/tree/v1 and https://codeocean.com/capsule/9050452/tree/v1Licensing provisions: MIT licenseProgramming language: FORTRANNature of problem: An initial constraint atomic configuration of a heterostructure is relaxed by imposing Keating's elastic interaction. The strain field and distribution of misfit dislocations are simulated for various quantum dot shapes and heterostructure materials.Solution method: The elastic energy of the heterostructure is minimized by an L-BFGS algorithm. The strain field is calculated by a discrete mathematical expression at each atomic location.

Combining synergetic effects of gradient nanotwins and nanoprecipitates in heterogeneous bronze alloy

Both strengthening effects of gradient nanostructure (especially the gradient nanotwins) and nanoprecipitates have been well-recognized to simultaneously improve the mechanical properties of metallic materials, e.g., the high strength–ductility synergy. In this study, a surface mechanical attrition treatment technique has been applied to fabricate a gradient nanostructured bronze alloy (Cu-12Sn-1.5Ni-0.6Fe) with pre-nanoprecipitates. Atomic-scale investigations were performed to capture the microstructural evolution governing the gradient refinement process along the depth direction of the treated alloy. On one hand, the body-centered cubic (bcc) Fe-rich pre-nanoprecipitates are found to be well-retained in the fabricated gradient nanostructured specimen. On the other hand, it is interesting to reveal that the semi-coherent interfacial zones between the bcc Fe-based nanoprecipitates and face-centered cubic (fcc) Cu matrix with the specific distribution of misfit dislocations could facilitate the nucleation and growth of gradient nanotwins along the depth direction. Specifically, the twinning partials could be associated with the dissociations of the misfit dislocations in the semi-coherent interfacial region, which is evidenced by the concomitant fcc-Cu/bcc-Fe/nanotwin triplex. As a result, the synergetic strengthening effects of gradient nanotwins and nanoprecipitates are combined to enhance the strength–ductility synergy and work hardening capacity. Moreover, in situ transmission electron microscope observations were conducted to understand the synergetic strengthening mechanisms of the gradient nanotwins and nanoprecipitates during the dynamic cracking process in the fabricated gradient nanostructured bronze alloy. The strategy of combining the nanoprecipitates and gradient nanostructure synergetic effects in our work might offer a novel pathway for designing high-performance metallic materials.

Distribution of Misfit Dislocations and Elastic Mechanical Stresses in Metamorphic Buffer InAlAs Layers of Various Constructions

The equilibrium distributions of the misfit dislocation density ρ(z) and elastic stresses e(z) are calculated along the direction of the epitaxial growth of the metamorphic InAlAs/GaAs(001) layer with higher In content (to 87 mol %) and various profiles of varying the composition: step, linear, and root. The calculations are performed using the method based on iteration searching for the minimum total energy of the system. It is shown that the largest differences between various constructions of the buffer layer are observed in the character of distributions ρ(z), rather than e(z). Unlike the traditional constructions with a step and linear gradients of the composition, which are characterized by a quite homogeneous distribution of misfit dislocations, in a buffer layer with a root composition gradient, the main part of such dislocations is concentrated in the lower part of the layer near the heteroboundary with a GaAs substrate, and their density sharply decreases by more than one order of value along the layer thickness, achieving the value minimum for all abovementioned constructions. In spite of the fact that the important effect of interacting the dislocations to each other is not taken into account in this work, the calculations enable us to establish the main peculiarities of the distributions ρ(z) and e(z) in various metamorphic buffer InAlAs layers, which were observed experimentally before. Thus, this approach can be effectively used when designing optimal constructions of the device metamorphic heterostructures.

Computational Analysis of Low-Energy Dislocation Configurations in Graded Layers

Graded layers are widely exploited in semiconductor epitaxy as they typically display lower threading dislocation density with respect to constant-composition layers. However, strain relaxation occurs via a rather complex distribution of misfit dislocations. Here we exploit a suitable computational approach to investigate dislocation distributions minimizing the elastic energy in overcritical constant-composition and graded layers. Predictions are made for SiGe/Si systems, but the methodology, based on the exact (albeit in two dimensions and within linear elasticity theory) solution of the stress field associated with a periodic distribution of defects, is general. Results are critically compared with experiments, when possible, and with a previous mean-field model. A progressive transition from one-dimensional to two-dimensional distributions of defects when continuous linear grading is approached is clearly observed. Interestingly, analysis of the low-energy distribution of dislocations reveals close analogies with typical pile-ups as produced by dislocation multiplication.

Bi-doped suppression of antisite disordering and associated magnetic properties of La2−xBixMnNiO6 (x = 0 and 1)

Here we report synthesis, structure, microstructure and magnetic properties of La2−xBixMnNiO6 (x = 0 and 1) double perovskites. Ricciardo et al (2009 Mater. Res. Bull. 44 239) have attempted to synthesize LaBiMnNiO6 (x = 1), but no further characterization was done due to large impurity content in the sample. We have been able to synthesize LaBiMnNiO6 phase at ambient pressure with traces of impurity at 750 °C using sol-gel method. This achievement leads us to compare the structural and magnetic properties of LaBiMnNiO6 with parent phase La2MnNiO6 to highlight the effect of Bi-doping in double perovskite. In contrast to the biphasic rhombohedral (R-3c) and monoclinic (P21/n) crystal structures of La2MnNiO6, LaBiMnNiO6 crystallized in single monoclinic (P21/n) phase. The EDX mapping confirmed the chemical homogeneity of the samples. The electron diffraction confirms the ordered structure of the sample. The microstructure analysis from HAADF-STEM revealed random distribution of misfit dislocations in the structure. Such defects are created to relax the strain due to unusual replacement of Mn/Ni atoms by La/Bi. We observed a decrease in TC with a large increase in magnetic moment of LaBiMnNiO6 compare to La2MnNiO6. There is also large suppression of low-temperature magnetic anomaly in Bi-substituted sample. The lowering of TC can be rationalized to the local structural distortion associated with the stereoactive 6s2-lone pair electron of Bi3+. On the other hand, the increase in magnetic moment and suppression of low-temperature magnetic anomaly for LaBiMnNiO6 can be ascribed to the suppression of antisite disorder in Bi-substituted sample.

The effect of a Sb and Ga intermediate layer on the interfacial layer properties of epitaxial GaSb on GaAs grown by metalorganic chemical vapor deposition

A method for the growth of high quality GaSb epilayer on GaAs (001) substrate is demonstrated in this study. It is found that a superior GaSb/GaAs interface can be obtained by depositing a thin Sb-precursor layer on the GaAs substrate containing a GaAs buffer layer. It is suggested that the growth occurs via the interface misfit (IMF) growth mode. Without this treatment, GaSb epilayer may grow according to the Stranski–Krastanov mechanism or via a blend of the Stranski–Krastanov and IMF mechanisms, leading to an inferior GaSb/GaAs interface. This could be due to the intermixing of anions, leading to the turbulent composition and misfit dislocation distribution at the heterointerface. It appears that with the addition of a Sb layer, IMF arrays can be formed at GaSb/GaAs interface resulting in superior GaSb layer without the need for changing the growth parameters.

Depth-resolved analysis of lattice distortions in high-Ge-content SiGe/compositionally graded SiGe films using nanobeam x-ray diffraction

We have investigated the three-dimensional configuration of lattice distortions, including lattice plane tilt and twist, in a high-Ge-content constant-composition Si0.3Ge0.7 (CC-SG)/compositionally graded SiGe strain-relaxed buffer (graded SRB)/Si(001) stacked structure. Position-dependent ω–2θ–φ mapping (or three-dimensional reciprocal space mapping) by synchrotron-based nanobeam x-ray diffraction revealed the in-plane distributions of both local tilt and twist within an area of 10 × 10 μm on the sample surface. Depth-resolved crystal information was extracted analytically on the basis of structural features in the graded SRB layer. As a result, a series of tomographic maps that show the three-dimensional distributions of tilt and twist around the CC-SG/graded SRB interface were obtained. Tomographic analysis indicates that the orientation of lattice planes in the graded SRB abruptly changes at a specific depth and at a specific interval. The misfit dislocation distribution observed using transmission electron microscopy is not homogeneous but concentrated at a specific depth, which accounts for the abrupt changes of lattice plane tilt and twist. Our tomographic results clearly verify the dislocation morphology in the SiGe stacked structure, which demonstrates that this analysis method can be a powerful tool for quantitative and non-destructive elucidation of a three-dimensional lattice structure with high spatial resolution.

Misfit dislocations induced by lithium-ion diffusion in a thin film anode

According to the diffusion kinetics and heteroepitaxial strained layer theory, this paper presents a theoretical model to investigate the generation and distribution of misfit dislocations in a thin film anode under galvanostatic and potentiostatic operations. The results show that the nucleation and density distribution of misfit dislocations largely depend on the thickness of the diffused layer and insertion time. When the thickness is less than a certain critical value, the total strain energy in the electrode is almost insusceptible and yet reduced by misfit dislocations for that of going beyond the critical value. Under potentiostatic operation, the rise range and response speed of the dislocation density are greater and faster. A certain region immediately possesses much lower dislocation density under the electrode surface compared with the region below it. These quantitative results can provide a new perspective into relieving diffusion-induced stress by misfit dislocations with the purpose of improving the mechanical durability of lithium-ion batteries.

Investigation of misfit dislocations in α-Fe2O3/α-Al2O3 interface by geometric phase analysis and dislocation density tensor analysis

The misfit dislocations in α-Fe2O3/α-Al2O3 heterostructure interfaces were investigated by high-resolution transmission electron microscopy (HRTEM), geometric phase analysis (GPA) and dislocation density tensor analysis. The misfit dislocations form a two-dimensional network structure at the interface. We characterized the atomic configurations and strain distribution of misfit dislocations by HRTEM and GPA. The observation indicates that one and two extra half planes/strain fields exist in the dislocation cores imaged along the [1¯100] and [112¯0] direction, respectively. Dislocation density tensor analysis gave a very high accuracy in determining the Burgers vectors and proved useful in accurately localizing the dislocation distribution in the core region. The results show that all the dislocations have the same Burgers vector, however the different space distributions of dislocation density, which may be attributed to the differences of atomic configuration in dislocation cores. Classical elasticity theory was found to be in agreement with the 3D visualizations of dislocation density tensor. The relationships between atomic configuration, dislocation density distributions, and strain distributions in dislocation cores were investigated, which can be described briefly as: one/two extra half planes induce one/two Burgers vectors in dislocation density distributions; one/two Burgers vectors induce one/two strain fields around dislocation core. Finally by comparing the experimental strain distributions with dislocation models (Peierls–Nabarro model and Foreman model), we found that all the dislocations follow the Foreman model (a=2), which indicates that they have the same spatial extension of strain field. This work demonstrates the superiority of dislocation density tensor analysis in the investigation of misfit dislocations, particularly the dislocations with complicated core structure.

Effects of dislocation strain on the epitaxy of lattice-mismatched AlGaInP layers

Strain fields arising from a non-uniform distribution of misfit dislocations in an underlying compositionally graded buffer are shown to be sufficiently strong to modify indium incorporation in III-phosphide light emitting layers. Composition fluctuations (xIn±0.02) in lattice-mismatched (AlyGa1−y)xIn1−xP thin films with length-scales of 5–10μm and a broadened light emission spectra are observed. Cathodoluminescence, photoluminescence and wavelength dispersive x-ray spectroscopies are used in this analysis to generate spatial maps of luminescence spectra and element distributions in metal-organic chemical vapor deposition (MOCVD) grown films. It is seen that these fluctuations due to misfit dislocations are hard to eliminate via growth-kinetics alone but can be lowered through the use of miscut substrates or spacer layers between the graded buffer layer and the active layer. A link between crosshatch surface-roughness and group-III atom distribution under group-V rich growth conditions in both AlInP and GaInP films is also demonstrated. In summary, the interaction of the dislocation strain field with the growth surface can affect the optical characteristics of lattice-mismatched LEDs even if the final threading dislocation density is low.

The misfit dislocation density profile in graded SiGe/Si(001) layers prepared at different temperatures

We present a generalization of the Dodson–Tsao kinematic model of misfit dislocation for graded SixGe1−x/Si(001) layers. The layers were prepared under different growing conditions (temperature). The misfit dislocation distribution has been determined by means of high-resolution x-ray scattering. Analysis of the reciprocal space maps was compared with the kinematic Dodson–Tsao model and the equilibrium Tersoff model.

Distribution of Misfit Dislocations at the InGaAs/GaAs(001) Interface Observed by Monochromatic X-Ray Topography

Anisotropies in misfit dislocations (MDs) at the interface of InGaAs/GaAs (001) are investigated by monochromatic X-ray topography (XRT) technique. Single MD line or several MD lines (MD bunching) are observed as white lines in XRT. Distribution, density, and number of MDs in one MD bunching are evaluated. The density of α-MDs is larger than that of β-MDs. Number of MDs in one MD bunching distribute with two peaks in both in-plane directions. In macroscopic view, α-MD bunching lay more orderly than the other. While in microscopic view, α-MDs gather for the range narrower than β-MDs.

Microstructural characterization of thick ZnTe epilayers grown on GaSb, InAs, InP and GaAs (1 0 0) substrates

This paper describes a comprehensive investigation of thick ZnTe epilayers (∼2.4 μm) grown under virtually identical conditions on GaSb, InAs, InP and GaAs (1 0 0) substrates using molecular beam epitaxy. Cross-section transmission electron micrographs of the different heterostructures showed greatly reduced defect densities away from the interface region. High-resolution electron micrographs revealed a highly coherent interface with isolated dislocations for the ZnTe/GaSb sample, and showed extensive areas with well-separated interfacial misfit dislocations for the ZnTe/InAs sample. Lomer edge dislocations with Burgers’ vector of (1/2) a 〈1 1 0〉, as well as perfect 60° dislocations, were identified at the interfaces of the ZnTe/InP and ZnTe/GaAs samples. Digital image processing based on lattice–fringe images was also used to analyze the spatial distribution of misfit dislocations at the hetero-interfaces, in particular to estimate the amount of residual strain.

Microstructure of Heteroepitaxial ZnTe Grown by Molecular Beam Epitaxy on Si(211) Substrates

The interface of ZnTe/Si(211) grown by molecular beam epitaxy was investigated by high-resolution transmission electron microscopy. Several types of defects such as misfit dislocations, stacking faults, agglomerations of vacancies, and precipitates were observed and studied by electron microscopy at the ZnTe/Si interface. The distribution of misfit dislocations at the interface was revealed with the assistance of the fast Fourier transformation filtering technique. A stick-and-ball interface model including misfit dislocation geometry is proposed. The possible origins of the stacking faults, vacancies, and precipitates are discussed.

Interface and defect structures in ZnO films on m-plane sapphire substrates

Interface and defect structures in ZnO films grown on ( 1 0 1 ¯ 0 ) m-plane sapphire substrates by plasma-assisted molecular beam epitaxy were investigated by transmission electron microscopy (TEM). Distribution of misfit dislocations (MDs) at the ZnO/Al 2O 3 interfaces was orientation dependent and quite anisotropic. In addition, type-І 1 stacking faults (SFs) bounded by the Frank dislocations with the Burgers vector of 1 / 6 [ 2 0 2 ¯ 3 ] were observed to be predominant. Many SFs initiated from the interface to the film surface were observed in addition to the MDs at the interface. The analysis of the high-resolution TEM revealed no correlation between the SFs and the MDs. The ZnO films were not found to be pure m-plane ZnO and ( 1 0 1 ¯ 3 ) domains were frequently observed in the film with otherwise mostly ( 1 0 1 ¯ 0 ) m-plane in nature. These ( 1 0 1 ¯ 3 ) domains were not nucleated at the interface but initiated inside the ZnO film above the interface and reached the top surface.

Relaxation of strained pseudomorphic SixGe1−x layers on He-implanted Si/δ-Si:C/Si(100) substrates

In this letter we present a method to increase the efficiency of SiGe layer relaxation by He+ ion implantation and annealing. Preferential nucleation of He platelets along a δ-impurity layer grown in the Si substrate below the SiGe layer results in planar localization and homogenization of dislocation loop sources inducing a more uniform distribution of misfit dislocations. We demonstrate this for a thin Si:C layer grown by reduced pressure chemical vapor deposition. The optimization of the conditions for efficient relaxation and layer quality is studied with respect to the position of the Si:C layer and the process parameters. Relaxation degrees up to 85% are obtained for Si0.77Ge0.23 layers.

Critical Thickness of Exponentially and Linearly Graded HgCdTe/CdZnTe

We have investigated the glide of strain-relaxing dislocations in closely lattice matched, liquid phase epitaxially (LPE) grown, HgCdTe. A generalized LPE heterostructure was modeled based on secondary-ion mass spectroscopy (SIMS) profile measurements. Critical thickness was predicted using a force balanced method which expands upon the work recently developed by Ayers. The behavior of dislocation dynamics is predicted with respect to exponentially and linearly graded metallurgical interfaces intrinsic to the high-temperature LPE growth process. The extended Ayers model is compared against x-ray topography and cross-sectional observations of misfit dislocations by the decoration of etch pits on cleaved HgCdTe/CdZnTe. The model predicts that, for bulk Cd/Zn compositions which are nearly lattice matched, the Zn compositional profile plays an important role in determining both the onset and distribution of misfit dislocations.

Relaxation of interphase stresses on the later stages of the heterogeneous decomposition of solid solutions. I. Functional of the energy

For describing the isomorphic heterogeneous decomposition of binary supersaturated solid solutions at the stage of coalescence under conditions of dislocation-matrix diffusion with a relaxation of stresses caused by phase separation, we consider a system representing a precipitation-phase particle in a surrounding matrix + dislocations intersecting the interphase surface and supplying an alloying component to it. In the approximations based on the continuity of the distribution of misfit dislocations on the interphase surface and of the average concentration field, we obtained an expression for the total energy of the system in the form of a functional which depends on the fraction of the relaxed regions of the interphase surface, the number of supplying (feeding) dislocations, and their mutual arrangement. The need for the use of a model functional appears within the framework of the variational problem for determining the equilibrium parameters of the system.

Misfit dislocation network in Cu/Ni multilayers and its behaviors during scratching

The structure and distribution of misfit dislocations at Cu–Ni interfaces and their effects on the tribological behavior of a Ni film are investigated with 3D Molecular Dynamic Simulations. The structure of misfit dislocation network at a Cu–Ni interface differs according to different crystallographic orientations of the film relative to the substrate: a triangle and square type of misfit dislocation network are observed at (111)Cu||(111)Ni and (001)Cu||(001)Ni interfaces respectively. They play an important role in the strengthening of Cu/Ni multilayers. During the scratching of a single asperity contact on the Ni film, the misfit dislocation network becomes a significant barrier to the glide dislocations. The plot of friction force vs. normal load when scratching on the Ni film exhibits a horizontal stage, representing the decreasing of the frictional coefficient due to the existence of the misfit dislocations network.

Structural impact of LPE buffer layer on sublimation grown 4H–SiC epilayers

The objective of this work was to study the effect of a liquid phase epitaxy buffer layer on the development of defects in sublimation grown epitaxial layers of 4H–SiC. The results were analyzed with the aid of optical microscope, scanning electron microscope, high-resolution X-ray diffraction and synchrotron white beam X-ray topography. A pronounced effect of the liquid phase epitaxy buffer layer on formation of dislocations and micropipes is observed in the sublimation epitaxy layers. It has been shown that during sublimation growth of epilayer with a thin liquid phase epitaxy buffer layer ( 0.1 μ m) defects may undergo transformation and stacking faults can be formed. Sublimation grown epilayers grown on a thick liquid phase epitaxy buffer layer ( 1 μ m) also showed a symmetrical distribution of misfit dislocations along the 〈1 1 2 ̄ 0〉 and [1 1 ̄ 0 0] directions.