Lomer Dislocations Research Articles

Transmission electron microscopy characterization of dislocation structure in a face-centered cubic high-entropy alloy Al0.1CoCrFeNi

Structural characterization of dislocations and dislocation reactions in a face-centered cubic high entropy alloy was conducted using the state-of-the-art spherical aberration corrected transmission electron microscopy. We experimentally measured the stacking fault energy of the high entropy alloy from the atomic images and diffraction contrast of dislocation cores. The low stacking fault energy results in widely dissociated dislocations and extensive dislocation reactions, which leads to the formation of immobile Lomer and Lomer-Cottrell dislocation locks. These dislocation locks act as both dislocation barriers and sources and are responsible for the significant work hardening with a large hardening rate in the alloy. Based on the atomic-scale characterization and classical dislocation theory, a simple equation was derived to describe the work hardening behavior of the high entropy alloy in the early-stage of plastic deformation.

Growth, structural, and electrical properties of germanium-on-silicon heterostructure by molecular beam epitaxy

The growth, morphological, and electrical properties of thin-film Ge grown by molecular beam epitaxy on Si using a two-step growth process were investigated. High-resolution x-ray diffraction analysis demonstrated ∼0.10% tensile-strained Ge epilayer, owing to the thermal expansion coefficient mismatch between Ge and Si, and negligible epilayer lattice tilt. Micro-Raman spectroscopic analysis corroborated the strain-state of the Ge thin-film. Cross-sectional transmission electron microscopy revealed the formation of 90 ° Lomer dislocation network at Ge/Si heterointerface, suggesting the rapid and complete relaxation of Ge epilayer during growth. Atomic force micrographs exhibited smooth surface morphology with surface roughness < 2 nm. Temperature dependent Hall mobility measurements and the modelling thereof indicated that ionized impurity scattering limited carrier mobility in Ge layer. Capacitance- and conductance-voltage measurements were performed to determine the effect of epilayer dislocation density on interfacial defect states (Dit) and their energy distribution. Finally, extracted Dit values were benchmarked against published Dit data for Ge MOS devices, as a function of threading dislocation density within the Ge layer. The results obtained were comparable with Ge MOS devices integrated on Si via alternative buffer schemes. This comprehensive study of directly-grown epitaxial Ge-on-Si provides a pathway for the development of Ge-based electronic devices on Si.

Plastic Deformation through Dislocation Saturation in Ultrasmall Pt Nanocrystals and Its in Situ Atomistic Mechanisms.

The atomic-scale deformation dynamic behaviors of Pt nanocrystals with size of ∼18 nm were in situ investigated using our homemade device in a high-resolution transmission electron microscope. It was discovered that the plastic deformation of the nanosized single crystalline Pt commenced with dislocation "appreciation" first, then followed by a dislocation "saturation" phenomenon. The magnitude of strain plays a key role on dislocation behaviors. At the early to medium stage of deformation, the plastic deformation was controlled by the full dislocation activities accompanied by the formation of Lomer dislocation locks from reaction of full dislocations. When the strain increased to a significant level, stacking faults and extended dislocations as well as Lomer-Cottrell locks appeared. The Lomer-Cottrell locks can unlock through transferring into Lomer dislocation locks first, and then Lomer dislocation locks were destructed under high stresses. The very high density dislocations and the frequent dislocation reactions through Lomer dislocations and Lomer-Cottrell locks may lead to work hardening in nanosized Pt.

Interactions between lattice dislocation and Lomer-type low-angle grain boundary in nickel

Symmetric tilt Lomer-type low-angle grain boundaries (LLAGBs) are distinct grain boundaries formed by Lomer dislocation locks in face-centered cubic (fcc) metals. To reveal their mechanical behaviors, interactions between lattice dislocation and LLAGB are studied with molecular dynamics simulations. Dislocation reaction and slip transmission depend on the tilt angle of LLAGB, the character of incident dislocation and the particular glide planes inhabiting the incoming slip. For LLAGBs with relatively small tilt-angles, a free slip-transmission zone can be identified where dislocations can be forced to penetrate through the LLAGB without inducing dislocation reaction. Otherwise, the incident slip tends to be trapped, triggering a number of dislocation reactions and leading to indirect slip transmission across the boundary in the reaction zone. Critical strains for dislocation reactions are examined. Raising up the temperature will widen the dislocation reaction zone and the free transmission zone tends to disappear.

The Relationship Between Atomic Structure and Strain Distribution of Misfit Dislocation Cores at Cubic Heteroepitaxial Interfaces

The atomic reconstruction of a misfit dislocation (MD) core causes change in the strain distribution around the core. Several MD cores at the AlSb/GaAs (001) cubic zincblende interface, including a symmetrical glide set Lomer dislocation (LD), a left-displaced glide set LD, a glide set LD with an atomic step, a symmetrical shuffle set LD, and a 60° dislocation pair, were studied using simulated projected potential and aberration-corrected transmission electron microscope images. Image deconvolution was also used to restore structure images from nonoptimum-defocus images. The corresponding biaxial strain maps, ε xx (in-plane) and ε yy (out-of-plane), were obtained by geometric phase analysis using the GaAs substrate as the reference lattice. The results show that atomic structure characteristics of MD cores can be revealed by the strain maps. The strain maps should be measured from optimum-defocus images or restored structure images. Furthermore, the ε xx strain map has been found more accurate than the ε yy strain map for MD cores, and the specimen thickness should be below the critical thickness due to the influence of dynamical scattering.

Effect of rate dependence of crack propagation processes on amorphization in Al

Uniaxial tension of pure Al is studied at different loading rates by molecular dynamics simulation to understand the initial nucleation processes for amorphous structures and twinning, especially, the competition between twinning partial dislocation and Lomer one. There exists a transition point KI=0.205eVÅ−2.5 between twinning partial dislocation and Lomer one, with Lomer dislocation preferable at high loads. A modified Peierls analytic model is established to describe the transition process. The analytic model reflects that a transition from twinning partial dislocation to Lomer one will occur when stress intensity factor KI reaches 0.205eVÅ−2.5, which is similar to the simulation trends. In addition, amorphization behaviours are observed at crack tip in pure Al by in-situ straining transmission electron microscopy. Both the experiment and simulation results are demonstrated that amorphization behaviours occur at crack tip in FCC Al.

Threading dislocation free GaSb nanotemplates grown by selective molecular beam epitaxy on GaAs (001) for in-plane InAs nanowire integration

The growth of in plane highly mismatched GaSb nanotemplates free of threading dislocations on GaAs (001) substrate is demonstrated using selective area growth. We report a detailed comparison of the crystalline quality of GaSb elaborated either as 2D layers or selectively grown inside 100nm wide stripes directed along [110] or [1–10]. By means of transmission electron microscopy, we show that the GaSb layer grown on a bare substrate contains an important density of threading dislocations, while the one grown inside nano-openings is free from threading dislocations. Moreover, we demonstrate that the GaSb layers in both growth modes are relaxed through a Lomer dislocation network at the interface, which is characterized using the full width at half maximum of the GaSb X-ray diffraction peak for different reflections. Finally, we demonstrate that these GaSb nanotemplates can be used for subsequent epitaxial growth of high structural quality strained InAs nanowires.

Atomic and electronic structure of Lomer dislocations at CdTe bicrystal interface.

Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a [1–10]/(110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocation cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. This report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis.

Atomic-column scanning transmission electron microscopy analysis of misfit dislocations in GaSb/GaAs quantum dots

The structural quality of GaSb/GaAs quantum dots (QDs) has been analyzed at atomic scale by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. In particular, we have studied the misfit dislocations that appear because of the high-lattice mismatch in the heterostructure. Our results have shown the formation of Lomer dislocations not only at the interface between the GaSb QDs and the GaAs substrate, but also at the interface with the GaAs capping layer, which is not a frequent observation. The analysis of these dislocations points to the existence of chains of dislocation loops around the QDs. The dislocation core of the observed defects has been characterized, showing that they are reconstructed Lomer dislocations, which have less distortion at the dislocation core in comparison to unreconstructed ones. Strain measurements using geometric phase analysis show that these dislocations may not fully relax the strain due to the lattice mismatch in the GaSb QDs.

Deformation mechanisms of bent Si nanowires governed by the sign and magnitude of strain

In this study, the deformation mechanisms of bent Si nanowires are investigated at the atomic scale with bending strain up to 12.8%. The sign and magnitude of the applied strain are found to govern their deformation mechanisms, in which the dislocation types (full or partial dislocations) can be affected by the sign (tensile or compressive) and magnitude of the applied strain. In the early stages of bending, plastic deformation is controlled by 60° full dislocations. As the bending increases, Lomer dislocations can be frequently observed. When the strain increases to a significant level, 90° partial dislocations induced from the tensile surfaces of the bent nanowires are observed. This study provides a deeper understanding of the effect of the sign and magnitude of the bending strain on the deformation mechanisms in bent Si nanowires.

Recombination at Lomer Dislocations in Multicrystalline Silicon for Solar Cells

Lomer dislocations at small-angle grain boundaries in multicrystalline silicon solar cells have been identified as responsible for the dominating inherent dark current losses. Resulting efficiency losses have been quantified by dark lock-in thermography to be locally up to several percent absolute, reducing the maximum power of the cells. By electron beam induced current measurements and scanning transmission electron microscopy investigations, it is revealed that the strengths of the dark current losses depend on the density of Lomer dislocations at the small-angle grain boundaries.

Do Lomer Dislocations Spoil High Performance of mc-Si Solar Cells?

Material induced inherent efficiency losses of multicrystalline silicon solar cells have been investigated across all scales from the solar cell down to the atomic structure of the responsible crystallographic defects. Material inherent efficiency losses can be attributed to local increased dark current, which is found at recombination active small angle grain boundaries and accounts to several per-cent absolute. Aone-to-one correlation between the density of Lomer dislocations and the strength of the recombination activity of small angle grain boundaries is found by electron-beam induced current measurements and scanning transmission electron microscope investigations. The increased recombination activity of Lomer dislocations is attributed to their immobile nature, which favors contamination by impurities.

Comprehensive molecular dynamics simulations of the stacking fault tetrahedron interacting with a mixed dislocation at elevated temperature

The defect-free channels were frequently observed in irradiated materials, i.e. copper, as a result of the stacking fault tetrahedron (SFT) interactions with dislocations. However, the underlying mechanisms for this process are still unclear to date. To address them, a comprehensive study on the interactions between SFTs and mixed dislocations was performed using molecular dynamics simulations. In particular, eight interaction geometries were considered, in terms of the dislocation Burgers vector directions, dislocation gliding directions and intersection positions on SFT. Various interaction outcomes were revealed after dislocation detachment. (1) SFT is fully absorbed through the transformation into Lomer dislocations, and subsequently moves out of free surfaces along the dislocation. (2) SFT is partially absorbed with the absorbed SFT base moving out of free surfaces along the dislocation. (3) SFT is not absorbed but sheared with ledges left on the SFT faces. (4) SFT is unaffected by the mixed dislocation. The current simulations, especially the full SFT absorption, provide important insights into the forming mechanisms of defect-free channels in irradiated materials.

The fracture behavior of twinned Cu nanowires: A molecular dynamics simulation

The molecular dynamics simulations are performed to explore the fracture behavior and the ductility of the twinned Cu nanowires containing orthogonally oriented growth CTBs due to the uniaxial tensile deformation. The results reveal that, the fracture behavior and the ductility of the twinned nanowires are not related to the length of the nanowires but also intensively related to the twin boundary spacing. When the twin boundary space is changed, the twinned nanowires undergo three distinct failure modes which include ductile fracture, brittle fracture and ductile-to-brittle transition depending on the length of the nanowires. We also find a reduction in the ductility of the twinned nanowires, which is ascribed to the deformation localization induced by the Lomer dislocation and the rapid necking resulted from the twinning partial slipping. Finally, the atomic-level process that occurs during deformation until final fracture are examined in detail, and a new formation mechanism of the Lomer dislocation is observed when a 90° partial dislocation transmits across a coherent twin boundary.

Assessment of phase-field-crystal concepts using long-time molecular dynamics

The ability of the phase-field-crystal (PFC) model to quantitatively predict atomistic defect structures in crystalline solids is addressed. First, general aspects of the PFC model are discussed within the context of obtaining quantitative results in solid materials. Then a specific example is used to illustrate major points. Specifically, accelerated molecular dynamics is used to compute the one-particle probability density rho((1))(r) in a complex atomistic defect consisting of a Lomer dislocation with an equilibrium distribution of vacancies in the core, and the results are considered within the general framework of the PFC model. As expected,.(1)(r) shows numerous spatially localized peaks with integrated densities smaller than unity, as would arise in a PFC computation. However, the rho((1))(r) actually corresponds to a time-averaged superposition of a few well-defined atomic configurations each having a well-defined energy. The deconvolution of rho((1))(r) to obtain the actual distinct atomic configurations is not feasible. Using a potential energy functional that accurately computes the energies of distinct configurations, the potential energy computed using rho((1))(r) differs from the actual average atomistic energy by similar to 50 eV divided among approximately 46 atoms in the core of the defect. Attempts to rectify this deviation by introducing correlations cannot significantly reduce this error. The simulations show energy barriers between distinct configurations varying by up to 0.5 eV, indicating that the simple kinetic evolution law used in PFC cannot accurately capture the true time evolution in this problem. Overall, these results demonstrate, in one nontrivial case, that the PFC model is probably unable to predict atomistic defect structures, energies, or kinetic barriers at the quantitative levels needed for application to problems in materials science.

Towards further understanding of stacking fault tetrahedron absorption and defect-free channels – A molecular dynamics study

The mechanisms leading to stacking fault tetrahedron (SFT) absorption via interactions with dislocations, and subsequent formation of plastic flow localization in defect-free channels, which were frequently observed in irradiated materials in transmission electron microscopy experiments, are still unclear. To address this, screw dislocation interactions with SFTs in copper were investigated using molecular dynamics (MD) simulations. The interaction details reveal that a screw dislocation can fully absorb an SFT through the thermally activated transformation of Lomer–Cottrell lock into Lomer dislocations. After absorption, almost all the vacancies in the SFT are transferred into Lomer dislocations, which are able to move transversely under complex loading conditions. As a result, SFTs can be removed from the material (for SFTs near surface) or from defect-free channels (for SFTs in the bulk) with the aid of Lomer dislocations. In addition, it was shown that this absorption process is favorable only at high temperature, low applied shear stress and/or high SFT density. These results are in good agreement with in situ TEM observations. The current simulations and analyses provide useful insights into the formation mechanisms of defect-free channels in irradiated materials.

Molecular dynamics simulation of tension–compression asymmetry in plasticity of fivefold twinned Ag nanopillars

Abstract Tension–compression asymmetry in plasticity is a notable feature in small-sized single crystals. Here we present results from molecular dynamics simulations of fivefold twinned Ag nanopillars in tension and compression. The fivefold twinned Ag nanopillar shows completely different deformation behaviors in each case. While surface nucleation of stacking fault decahedrons and subsequent formation of necking at region with high density of stacking fault decahedrons is the primary deformation mechanism in tension, generation of { 001 } 〈 110 〉 Lomer dislocations and their subsequent cross-slip are observed in compression. The reason for the difference is simply discussed by considering the conditions of activation of trailing partials.

Mechanical behaviors of single crystalline and fivefold twinned Ag nanowires under compression

The mechanical behaviors of fivefold twinned (FT) Ag nanowires under compression are investigated by molecular dynamics simulations. As a comparison, the single crystalline (SC) nanowires are also investigated. The fivefold twin boundary strengthens not only the tensile yield strength but also the compressive yield strength, suggesting the initial stress distribution is not the main factor behind the strengthening. The modulus-based explanation is also found invalid since similar modulus is observed between these two types of nanowires. In contrast to “dislocation starvation” state observed in SC nanowires under compression, the addition of fivefold twin boundary into nanowires leads to complex dislocation–dislocation and dislocation–twin interactions, contributing to increased dislocation density. While extend dislocation slip dominates the plastic deformation in SC nanowires, generation of {001}〈110〉 Lomer dislocations and their subsequent cross-slip are found to be the dominant deformation mechanisms in FT Ag nanowires under compression. Effects of boundary condition and sample geometry on plastic deformation behaviors are also investigated.

Investigation on the Formation and Propagation of Defects in GeSn Thin Films

Defect formation and propagation in Ge1-xSnx/Ge thin films were investigated by using X-ray diffraction, transmission electron microscopy and scanning electron microscopy techniques. Samples with 0.9 to 7 % Sn mole fraction were deposited on a Si substrate with a Ge buffer layer using a chemical vapor deposition technique. The X-ray diffraction results show that the Ge1-xSnxfilms are 46-100 % compressively strained. Transmission electron microscopy images show that symmetrical and asymmetrical Lomer dislocations are formed at GeSn/Ge interface, which stops propagation of defects parallel to the growth direction. Average threading dislocation density (TDD) in the Ge buffer layer is measured to be 9.71×108 cm-2. Adding Sn to Ge made the Ge1-xSnx films to be more flexible, which traps 80 % of TDD propagating in Ge buffer layer. Reduction of TDD to 3×107cm-2 indicates achievement of higher quality of Ge1-xSnx film.

Investigation on the Formation and Propagation of Defects in GeSn Thin Films

Silicon photonics is rapidly growing because of the recent introduction of GeSn alloys as a group IV direct bandgap material. Viability of GeSn alloys for complementary metal-oxide-semiconductor technology, as demonstrated recently, is another step towards commercializing this material for integrated circuits applications [1]. However, ability to grow high quality materials is imperative for device applications. In order to compensate the high lattice mismatch between the Si substrate and the GeSn film different strain relaxed buffer layers, such as Ge, are used. Relaxation of Ge buffer layer introduces a network of misfit dislocations. This can result in high threading dislocation density (TDD) in GeSn degrading the performance of the final device. In this paper, we report on defect formation and propagation mechanism in Ge1-xSnx films.Plastic relaxation of higher lattice constant materials (SiGe, Ge, GeSn) on top of Si forces the layers to form 60o misfit dislocations. These dislocations are caused by slip of (110) and (111) planes in the diamond lattice structure, which have a screw component that threads through the grown film. Fig. 1 (a) schematically shows how defects form and propagate through the film. A network of misfit dislocations results in a cross-hatch pattern on the surface of the film. Fig. 1 (b) shows the plan view transmission electron microscopy (TEM) image of Ge.93Sn.07 on top of Ge buffer layer. The cross-hatch patterns formed at GeSn/Ge interface have overlapped with those of Ge/Si. The denser pattern is attributed to the Ge/Si interface due to higher lattice mismatch. Fig. 1 (c) delineates scanning electron microscopy image of decorated threading dislocations at the surface by using cold Schimmel solution (CrO3+HF) for 10 minutes. Density of pits (1.1×107 cm-2) is in good agreement with TDD measured from TEM micrographs (3×107 cm2)[1]. It also confirms that the quality of GeSn material is higher than Ge buffer layer (TDD: 9.71×108 cm-2) by an order of magnitude.Formation of Lomer misfit dislocaltion prevents propagation of threadig dislocation in the top layer.The main reasons behind this phenomenon is that the Burgers vector in Lomer misfit dislocation, for a (001) substrate, lies parallel to the interface [2]. Therefore, the substrate and film lattices can match while no prependicular component exists. In this case the film is grown relaxed without introducing structural imperfection such as threading dislocations, which degrades the material quality. Different models of Lomer misfit dislocation are proposed including a five-atom ring and seven-atom ring without dangling bonds [3]. Available models show that Lomer dislocations can combine two lattices through symmetric and asymmetric loops [4]. Fig. 1 (d-g) shows formation of the symmetric and asymmetric loops at Ge/GeSn interface films using high resolution TEM images.Our investigations show that formation of symmetric and asymmetric Lomer dislocation at the interface of Ge/GeSn film has reduced the TDD in GeSn film. Results from TEM images and etch pit density measurement verifies this conclusion by showing that 80% of the threading dislocations has not passed through the GeSn film. The current results show achievement of high quality GeSn film as well as indicating that GeSn is a unique candidate to be used as a universal compliant layer.