Misfit Dislocation Structure Research Articles

Effects of interdiffusion on shear response of semi-coherent {111} interfaces in Ni/Cu

Intermixing of chemical species as a result of interdiffusion during the manufacturing of metallic nanolaminates leads to diffuse interface structures which have distinct properties compared to corresponding atomically sharp interfaces. The effect of interdiffusion-induced changes to the interface structure on interface shear strength is complex due to the presence of solute atoms and a reduced misfit dislocation density. The shear responses of {111} Cu/Ni nanolaminate interfaces with varying levels of interdiffusion are studied using atomistic methods to elucidate the effect of interface structure changes on shear deformation mechanisms. Models with diffuse interfaces exhibit improved interface shear strength relative to the atomically sharp case; however, shear strength does not increase monotonically with solute concentration. The distribution of maximum changes in energy per misfit node and non-uniform misfit node displacements, filtered using microrotation vector analysis, suggest heterogeneous interface resistance to sliding. No strong correlation is found between solute concentration near misfit node centroids and misfit node displacements, indicating the importance of the longer-range misfit dislocation structure. Increased activation of misfit dislocation glide is associated with larger solute concentrations as a result of increased misfit node displacements. Analysis of change in energy during the shear deformation process, however, reveals that interface sliding is dominated by the misfit node behavior. These findings highlight the importance of modeling realistic diffuse interface structures and emphasize the competing effects of solute concentration and interface misfit dislocation density.

Reduced dislocation growth leads to long lifetime InAs quantum dot lasers on silicon at high temperatures

We describe the effectiveness of filter layers, which displace misfit dislocation (MD) formation away from the active region, in improving high temperature reliability of epitaxially integrated InAs quantum dot lasers on on-axis silicon substrates. We find that inserting these “trapping layer (TL)” filters at either 80 nm or 180 nm from the active region substantially reduces device degradation at 60 °C. After 3000 h of continuous operation, the best trapping-layer-free device shows a 55% increase in threshold current while the best trapping layer (TL) devices each show less than a 9% increase. We explain these findings by correlating changes in individual device performance to changes in misfit dislocation (MD) structure. All MDs in devices without TLs show evidence of recombination enhanced dislocation climb (REDC); in contrast, adding trapping layers at 180 nm or 80 nm reduces the fraction of electrically active MDs to 9% and 1%, respectively. Reliability data after 3000 hours suggest that incorporating trapping layers a shorter distance from the active region (80 nm) is more effective than incorporating these layers further away. We conclude by identifying the mutually and self-reinforcing failure processes associated with REDC that TLs significantly remediate: increasing dislocation line length, increasing point defect densities, and increasing junction temperature. Overall, understanding and controlling crystal defects continues to be the most impactful avenue toward integrating light sources on photonic integrated circuits and closing the gap with native-substrate lasers.

Lattice dislocation induced misfit dislocation evolution in semi-coherent {111} bimetal interfaces

The study of dislocation plasticity mediated by semi-coherent interfaces can aid in the design of certain heterostructured materials, such as nanolaminates. The evolution of interface misfit patterns under complex stress fields arising from dislocation pileups can influence local dislocation/interface interactions, including effects of multiple incoming dislocations. This work utilizes the Concurrent Atomistic-Continuum modeling framework to probe the evolution of misfit structures at semi-coherent Ni/Cu and Cu/Ag interfaces impinged by dislocation pileups generated via nanoindentation. A continuum microrotation metric is computed at various stages of the indentation process and used to visualize the evolution of the interface misfit dislocation pattern. The stress state from approaching dislocations induces mixed contraction and expansion of misfit dislocation structures at the interface. A lower number of misfit nodes per unit interface area coincides with greater localized deformation with regard to atoms near misfit nodes for Ni/Cu. The decreased misfit node spacing for Cu/Ag alternatively distributes the restructuring associated with plastic deformation over a larger percentage of atoms at the interface. Interface sliding facilitated by misfit dislocation motion is found to facilitate deformation extending into the bulk lattices centered on misfit nodes. The depth of penetration of those fields is found to be greater for Ni/Cu than for Cu/Ag. The study of dislocation plasticity mediated by semi-coherent interfaces can aid in the design of certain heterostructured materials, such as nanolaminates. The evolution of interface misfit patterns under complex stress fields arising from dislocation pileups can influence local dislocation/interface interactions, including effects of multiple incoming dislocations. This work utilizes the Concurrent Atomistic-Continuum modeling framework to probe the evolution of misfit structures at semi-coherent Ni/Cu and Cu/Ag interfaces impinged by dislocation pileups generated via nanoindentation. A continuum microrotation metric is computed at various stages of the indentation process and used to visualize the evolution of the interface misfit dislocation pattern. The stress state from approaching dislocations induces mixed contraction and expansion of misfit dislocation structures at the interface. A lower number of misfit nodes per unit interface area coincides with greater localized deformation with regard to atoms near misfit nodes for Ni/Cu. The decreased misfit node spacing for Cu/Ag alternatively distributes the restructuring associated with plastic deformation over a larger percentage of atoms at the interface. Interface sliding facilitated by misfit dislocation motion is found to facilitate deformation extending into the bulk lattices centered on misfit nodes. The depth of penetration of those fields is found to be greater for Ni/Cu than for Cu/Ag.

New insight into prismatic-type face-centered cubic zirconium phase in pure zirconium

Distinctive thermal-induced prismatic-type face-centered cubic zirconium phases (named γ phase) were firstly reported in pure zirconium. Atomic insight was provided into the α/γ interface and structure of interfacial misfit dislocations. Furthermore, we proposed the formation mechanism of this γ phase. Our results revealed that the semi-coherent interface contributes to periodic extra half-plane on the γ side and the volume expansion is 19.8% due to the α → γ phase transformation. More importantly, the α → γ phase transformation involved expansion and shuffle displacement.

Interface structure and deformation mechanisms of AlN/GaN multilayers

Experimental results show that nano-ceramic/ceramic multilayers have excellent properties, and the deformation mechanisms need further investigation. In this work, molecular dynamics simulations were performed to study the interface structure of an AlN/GaN multilayer as well as the deformation mechanisms under uniform tensile/compressive loadings. After relaxation, a hexagonal misfit dislocation structure was observed at the interface. Analysis revealed that the misfit dislocation lines, surrounded by coherent regions, are of edge type. Unlike metal/metal multilayers and metal/ceramic multilayers, the stress-strain curve of the AlN/GaN multilayer subjected to in-plane loading shows only single main yield point. This is because that dislocations nucleate from the interface and emit into both layers almost simultaneously. A rhombus network was observed in GaN layer, attributed to the slip of dislocations along [112‾0](11‾00) and [21‾1‾0](011‾0) in wurtzite lattice structure. The tension-compression asymmetry was also found in the AlN/GaN multilayer.

Beyond Coherent Oxide Heterostructures: Atomic‐Scale Structure of Misfit Dislocations

AbstractNanoscale design of complex oxide heterostructures and thin films is imperative as they have significant promise in novel technological applications. A coherent interface is formed in oxide heterostructures with small mismatches, and the lattice mismatch is completely compensated by elastic strain. In semi‐coherent oxide heterostructures, when an epitaxial layer is grown on the substrate above the critical thickness of the film, misfit dislocations are formed to mitigate the strain between the two materials with dissimilar lattice constants. Key properties of semi‐coherent oxide heterostructures are influenced or even controlled by the presence of misfit dislocations. Therefore, it is critical to understand the atomic‐scale structure of semi‐coherent oxide heterostructures, specifically the structure of misfit dislocations that are ubiquitous at such heterointerfaces. Numerous state‐of‐the‐art experiments have reported emergent phenomena at semi‐coherent oxide heterostructures, wherein misfit dislocations play a crucial role. However, their atomic‐scale and nanoscale structure is not always discernable from experiments. Due to large system sizes, computational studies dedicated to examining misfit dislocations in semi‐coherent oxide heterostructures are still in their infancy. This review aims to summarize the recent advancements and challenges involved in computational studies elucidating the atomic‐scale structure of misfit dislocations in semi‐coherent oxide heterostructures and motivate future computational efforts.

A multiscale study of misfit dislocations in PbTe/PbSe(001) heteroepitaxy

Abstract

Stronger and more failure-resistant with three-dimensional serrated bimetal interfaces

Low-energy structures of bimetal interfaces commonly occur in nature, yet higher energy forms, made by deviations in the interface plane, are also likely. While these variants may occur less frequently, they can still play an important role in the response of the material under deformation, by acting as preferential sites for defect formation and boundary motion. Here, using atomic-scale simulation and interface defect theory and considering two bimetal systems, Cu/Ag and Cu/Nb, we show that high-energy interfaces can achieve a local, low-energy state by forming atomic-scale serrations and that the predicted size and location of the serrations are consistent with experimental observation. For several distinct strain states, we reveal that interfaces with atomic-scale serrations bear both higher barriers for dislocation nucleation and higher resistances to interfacial shear compared to their planar interface variants. This desirable combination is not a characteristic of the more commonly studied low-energy interfaces, which typically possess a high nucleation barrier and low sliding resistance. We explain, through an analysis of the misfit dislocation structure, that the serrations alter the number of dislocations emitted, change the favorable slip systems, and alleviate the stress concentrations generated by misfit dislocations. An interface engineering strategy is then proposed for designing atomically serrated interfaces to improve mechanical strength and hinder localization and creep of metallic nanomaterials.

Influence of Chemistry and Misfit Dislocation Structure on Dopant Segregation at Complex Oxide Heterointerfaces

AbstractComplex oxide heterostructures and thin films have emerged as promising candidates for diverse applications. Lattice mismatch between the two oxides lead to the formation of misfit dislocations, which influence vital material features. Trivalent dopant segregation to misfit dislocations at semi‐coherent oxide heterointerfaces, while not well understood, is anticipated to impact interface‐governed properties. Here, atomistic simulations elucidating the influence of misfit dislocations on dopant segregation at SrTiO3/MgO heterointerfaces are reported. SrO‐ and TiO2‐terminated interfaces that have differing misfit dislocation structure were considered for trivalent dopants segregation. At SrO‐terminated interface, dopants tend to segregate toward but not precisely to the heterointerface, whereas at TiO2‐terminated interface, dopants exhibit a thermodynamic preference to accumulate at the heterointerface. Most favorable segregation sites at SrO‐terminated interface are located within the coherent terrace, whereas those at TiO2‐terminated interface are at misfit dislocation intersections. Atomic layer chemistry and the resulting misfit dislocation structure at the heterointerface, along with concomitant strain at the heterointerface due to mismatched dopants, play a critical role in influencing the observed trends for dopant segregation. Overall, the present results offer a fundamental atomic scale perspective of dopant behavior at semi‐coherent complex oxide heterointerfaces and the interplay between dopant chemistry, interface chemistry, and misfit dislocation structure.

Core structure and strengthening mechanism of the misfit dislocation in nickel-based superalloys during high-temperature and low-stress creep

Misfit dislocation structures are investigated in a nickel-based single crystal superalloy subjected to 38.8 h creep at 1100 °C and 130 MPa using conventional and high resolution transmission electron microscopy (HRTEM). The misfit dislocation was identified as an uncommon one with Burgers vector b = a/2 [2¯11], which can be described as inserting (or extracting) one (11¯1) half plane and one (111¯) half plane. This dislocation was decomposed into two partial dislocations b1 and b2, with b1 = a/2[1¯01] and b2 = a/2[1¯10]. Partial dislocations b1 and b2 were arranged in a “V” shape, connected by the antiphase boundary (APB), which could hinder the dislocation motion to achieve the effect of dislocation enhancement.

The Misfit Dislocation Core Phase in Complex Oxide Heteroepitaxy

AbstractMisfit dislocations form self‐organized nanoscale linear defects exhibiting their own distinct structural, chemical, and physical properties which, particularly in complex oxides, hold a strong potential for the development of nanodevices. However, the transformation of such defects from passive into potentially active functional elements necessitates a deep understanding of the particular mechanisms governing their formation. Here, different atomic resolution imaging and spectroscopic techniques are combined to determine the complex structure of misfit dislocations in the perovskite type La0.67Sr0.33MnO3/LaAlO3 heteroepitaxial system. It is found that while the position of the film–substrate interface is blurred by cation intermixing, oxygen vacancies selectively accumulate at the tensile region of the dislocation strain field. Such accumulation of vacancies is accompanied by the reduction of manganese cations in the same area, inducing chemical expansion effects, which partly accommodate the dislocation strain. The formation of oxygen vacancies is only partially electrically compensated and results in a positive net charge q ≈ +0.3 ± 0.1 localized in the tensile region of the dislocation, while the compressive region remains neutral. The results highlight a prototypical core model for perovskite‐based heteroepitaxial systems and offer insights for predictive manipulation of misfit dislocation properties.

Characterization of the misfit dislocations at the ferrite/cementite interface in pearlitic steel: An atomistic simulation study

The characteristics of the misfit dislocations at ferrite/cementite interfaces (FCIs) for various orientation relationships (ORs) have important implications for the mechanical behavior and the phase transformation of pearlitic steels; however, the detailed characteristics of these misfit dislocations have not been thoroughly elucidated to date. Using the extended atomically informed Frank–Bilby (xAIFB) method and atomistic simulation, we characterized the structures of misfit dislocations and calculated the interface energies of five ORs (Bagaryatsky, Isaichev, Pitsch–Petch, Near Bagaryatsky and Near Pitsch–Petch), respectively. Atomistic calculations of the interface energies of five ORs reveal that (1) the Isaichev OR has the lowest interface formation energy and (2) Near Bagaryatsky and Near Pitsch–Petch ORs are energetically more favorable than exact Bagaryatsky and Pitsch–Petch ORs in spite of small misorientation angle. The interface formation energy of each OR is qualitatively well explained by the structure and spacing of FCI dislocations, which demonstrate the importance of the characterization of misfit dislocations.

Antimony-Mediated Control of Misfit Dislocations and Strain at the Highly Lattice Mismatched GaSb/GaAs Interface

Determining the atomic structure of misfit dislocations at highly lattice mismatched interface is essential to optimize the quality of the epitaxial layer. Here, with aberration corrected scanning transmission electron microscopy at sub-Angstrom resolution and molecular dynamics simulation, we investigated the atomic structure of misfit dislocations at GaSb/GaAs interface. New types of Lomer misfit dislocation formed on an Sb wetting monolayer were observed, in contrast to a conventional misfit dislocation whose core is located at interface. These Sb-mediated dislocations have highly localized cores and offer more capability to confine the mismatch strain at the interface. The low strain atomic configuration of Sb-mediated dislocations is driven by minimization of the core energy. This unveiled mechanism may pave the way to the growth of high quality hetero-epitaxial layers.

Transmission electron microscopy characterization of CrN films on MgO(001)

Two CrN(001) films with different thickness were grown on MgO(001) substrates using unbalanced d.c. magnetron sputtering. The morphology and interfacial structure of the films are characterized by using conventional transmission electron microscopy, weak-beam dark-field microscopy and spherical aberration (CS)-corrected high-resolution transmission electron microscopy. The microscopy studies revealed the well-known cube-on-cube orientation relationship. While an interface dislocation network with b→=½aCrN<100> edge dislocations was identified, only part of the lattice mismatch is relaxed. The misfit dislocation structure and growth defects are analyzed and discussed based on the weak-beam dark-field and high-resolution transmission electron microscopy results.

Mechanism of induced nucleation of misfit dislocations in the Ge-on-Si(0 0 1) system and its role in the formation of the core structure of edge misfit dislocations

Ge-on-Si(001) films are grown by molecular beam epitaxy via a three-step epitaxial growth method (Ge/Ge seed/GeSi buffer/Si(001)). The dislocation structure of the Ge/GeSi buffer interface is studied by high-resolution electron microscopy. Misfit dislocations on the interface are edge dislocations and are aligned regularly with a period of 9–10nm. A variety of atomic structures of the dislocation core is observed, known in the literature as dissociated or asymmetric Lomer edge dislocations. The assumption that atomic structures of various degrees of complexity are intermediate states in the formation of a perfect edge misfit dislocation in the course of plastic relaxation of a stressed film is justified. A model is proposed which explains the intermediate states in terms of statistical variation of the nucleation site of the complementary 60° dislocation which forms, together with the primary dislocation, a Lomer dislocation at the interface.

Strain rate sensitivity and activation volume of Cu/Ni metallic multilayer thin films measured via micropillar compression

Micropillar compression testing with repeated jumps in strain rate is used to circumvent inherent difficulties associated with nanoindentation and tensile testing of free-standing films. Application to sputtered 21 nm/21 nm Cu/Ni multilayer thin films with a cube-on-cube texture reveals an average strain rate sensitivity (m = 0.014) and activation volume (V = 17 b3), comparable to nanocrystalline face-centered cubic metals. Yet, m increases by ∼50% and V decreases by 70% with increasing strain, opposite to trends reported for nanotwinned Cu. The large, strain-dependent shifts in m and V are dependent on the underlying misfit dislocation structure of Cu/Ni interfaces.

Molecular dynamics simulations of glide dislocations induced by misfit dislocations at the Ni/Al interface

Abstract 3D molecular dynamics simulations were employed to investigate the influence of misfit dislocations of Ni–Al interface on glide dislocations under uniaxial loading for some specific crystallographic orientations. The simulation results show that the glide dislocations nucleated from the interface were dependent on the misfit dislocation structure at the Ni–Al interface. Square, disordered and polyline types of dislocations were observed at Al(0 0 1)/Ni(0 0 1), Al(0 0 1)/Ni(1 1 0) and Al(0 0 1)/Ni(1 1 1) interface, respectively. Moreover, under uniaxial loading, glide dislocations were induced from the line of misfit dislocations at the interface, and due to the hard–soft interface structure, glide dislocations were only observed in Al. Glide dislocations mainly occurred on {1 1 1} planes, and when these dislocation planes expanded they might collide with each other.

High-resolution electron microscopy of misfit dislocations in AlSb/GaAs（001） system

The detailed core structures of misfit dislocations in the AlSb/GaAs（001） heterostructure system were studied by 200 kV LaB6 filament high-resolution electron microscope. In combination with image deconvolution, the［110］ images were transformed into the projected structure maps, and the image resolution was enhanced up to the information limit of the microscope. To distinguish Al and Sb atoms in the AlSb film, the image contrast change with the sample thickness was analyzed for the perfect region in deconvoluted image, and the positions of Al and Sb atoms in the dumbbells were determined based on the image contrast theory of the pseudo-weak-phase object approximation. Then the structure models of two types of misfit dislocations were constructed. As the simulated images are in good agreement with the experimental images, the AlAs type interface and the core structures of obtained Lomer and 60° misfit dislocations were determined.

Partial strain relaxation via misfit dislocation generation at heterointerfaces in (Al,In)GaN epitaxial layers grown on semipolar (112¯2) GaN free standing substrates

Misfit strain relaxation via misfit dislocation (MD) generation was observed in heteroepitaxially grown (Al,In)GaN layers on free-standing semipolar (112¯2) GaN substrates. Cross-section transmission electron microscope images revealed MD arrays at alloy heterointerfaces, with the MD line direction and Burgers vector parallel to [11¯00] and [112¯0], respectively. The MD structure is consistent with plastic relaxation by dislocation glide on the (0001) plane. Since (0001) is the only slip plane, the plastic relaxation is associated with tilt of the epitaxial (Al,In)GaN layers. The tilt, measured via high-resolution x-ray diffraction, can be used to quantify the relaxation.

Molecular dynamical investigation on plastic behavior of Cu(100) twist-grain boundary under uniaxial tension

Misfit dislocation structures at twist-grain boundary and its effect on grain boundary strength are investigated by using molecular dynamics simulation with EAM potential. The results reveal that the density of misfit dislocations which are formed on small angle twist-grain boundary is increased as the twist angle increased. Dislocations nucleate at each unit of the dislocation networks and the yield stress is increased as the density of misfit dislocations increases since grain boundary is strengthened by the interaction between dislocation networks. Face defect is formed at large angle twist-grain boundary, and the dislocations nucleate at the atomically sharp corner of the grain boundary. The yield stress is kept almost at the same value since there is little effect on the nucleation force of dislocation on face-defect grain boundary.