Electron Channeling Contrast Imaging Research Articles

Non-destructive imaging of residual strains in GaN and their effect on optical and electrical properties using correlative light–electron microscopy

We demonstrate a non-destructive approach to understanding the growth modes of a GaN thin film and simultaneously quantify its residual strains and their effect on optical and electrical properties using correlative scanning electron microscopy techniques and Raman microscopy. Coincident strain maps derived from electron backscatter diffraction, cathodoluminescence, and confocal Raman techniques reveal strain variations with similar magnitude and directions, especially in the proximity of dislocations. Correlating confocal Raman imaging with electron channeling contrast imaging suggests that the dislocations organize themselves to form a distinctive pattern as a result of the underlying growth mask, where some of them align along the [0001] growth direction and some are inclined. The methodology presented in this work can be adopted to investigate any heteroepitaxial growth, in particular, those using selective masks on the growth substrates, where the morphology influences the subsequent growth.

Investigation of the effect of structural defects from hydride precipitation on superconducting properties of high purity SRF cavity Nb using magneto-optical and electron imaging methods

Nb is an elemental superconductor with a critical temperature of 9.3 K and is widely used to fabricate superconducting radiofrequency (SRF) cavities for particle accelerators. However, microstructural defects in Nb, such as grain boundaries (GBs) and dislocations, can act as pinning centers for magnetic flux that can degrade SRF cavity performance. Hydrogen contamination is also detrimental to SRF cavity performance due to the formation of normal conducting hydrides during cool down. In this study, disc shaped Nb bi-crystals extracted from high-purity large-grain Nb slices were investigated to study the effects of GBs, hydrogen, and dislocations on superconducting properties. Grain orientation and GB misorientation were measured using Laue x-ray diffraction and electron backscattered diffraction (EBSD) analyses. Cryogenic magneto-optical imaging was used to directly observe magnetic flux penetration below = 9.3 K. Damage caused by low temperature precipitation of hydrides and their dissolution upon reheating after cryogenic cycles was examined using electron channeling contrast imaging, and EBSD. The relationships between hydride formation, dislocation content, GBs, cryo-cooling, heat treatment (HT), and flux penetration indicate that both GB character and hydrogen content affect magnetic flux penetration. Such flux penetration could be facilitated by dislocation structures and low angle GBs resulting from hydride precipitation and HT.

Laser-clad Nb(Ta)TiZr medium-entropy alloy coatings on pure Zr sheet: Microstructural characteristics, hardness and wear resistance

In this work, two Nb(Ta)TiZr medium-entropy alloy (MEA) coatings were fabricated successfully on a commercially pure Zr sheet by pulsed laser cladding. Their phase constitutions, microstructural characteristics, chemical compositions and grain orientations were well characterized and analyzed by using X-ray diffraction (XRD), electron channeling contrast (ECC) imaging, energy dispersive spectrometry (EDS) and electron backscatter diffraction (EBSD) techniques, with hardness and wear resistance measured and correlated with the microstructural characteristics as well. Results show that irregular-shaped bulk grains (BCC solid-solution phase) are formed in the two Nb(Ta)TiZr MEA coatings (cladding zones), both of which have a thickness of ∼800 μm with good metallurgical bonding to the substrate. After adding Ta, grains in the MEA coating are slightly refined. The phase constitutions in both the coatings agree with calculations based on several theoretical parameters. Hardness tests show that the NbTiZr and the NbTaTiZr coatings have hardnesses of 360 ± 18 HV and 430 ± 28 HV, respectively, ∼1.8 times and ∼2.2 times that of the substrate (196 ± 4 HV). This can be ascribed to combined effects of grain refinement strengthening, solid solution strengthening with denser low angle boundaries in the MEA coatings. Specific wear rates of the NbTiZr and the NbTaTiZr coatings are only ∼17% and ∼6% of that of the substrate, respectively, indicating markedly improved wear resistance. This is demonstrated to be closely related to complete oxide films easily formed on the coatings with oxidation and abrasive wear mechanisms.

Defect characterization of {101¯3} GaN by electron microscopy

Advances in obtaining untwinned (101¯3)-oriented semi-polar GaN enable a new crystal orientation for the growth of green and red LED structures. We present a scanning electron microscopy study that combines the structural characterization of electron channeling contrast imaging with the optical characterization of cathodoluminescence hyperspectral imaging on a (101¯3) GaN layer. An extensive defect analysis revealed that the dominant defects consist of basal plane stacking faults (BSFs), prismatic stacking faults, partial dislocations, and threading dislocations. With a defect density of about an order of magnitude lower than in comparable. The optical properties of the defects have been characterized from 10 to 320 K, showing BSF luminescence at room temperature indicating a reduced density of non-radiative recombination centers in the as-grown samples compared to established semi- and non-polar orientations. Our findings suggest that growth along (101¯3) has the potential for higher radiative efficiency than established semi-polar orientations.

Multi-Probe Characterization of Plasticity Heterogeneity: Requirement of Alloy Design Considering Dislocation Planarity for Developing Crack-Resistant Ni–Cr Alloys

Ni–20Cr alloy is a representative model alloy that shows planar dislocation motion. The planar dislocation motion causes plasticity-induced stress concentration at grain boundaries, which can be a reason for intergranular cracking. In this study, the micro-deformation behavior in Ni–20Cr alloy and its interaction with grain boundaries were investigated. To clarify the micro-plasticity near grain boundaries in Ni–20Cr alloy, we present an applicability of the in situ electron channeling contrast imaging (ECCI) method coupled with in situ micro-deformation analysis, digital image correlation (DIC), and post-mortem electron backscatter diffraction (EBSD). The in situ multi-probe microscopy showed planar dislocation pile-up and local dislocation accumulation, which caused local stress concentration at the grain boundaries. The degree of plasticity-enhanced stress concentration appeared most distinctly near grain boundary triple junction. This result is consistent with previous studies that reported brittle-like cracking can preferentially occur at the grain boundary triple junction. That is, the plasticity-enhanced stress concentration can play a role in brittle-like intergranular cracking that occurs after significant plastic deformation under some special conditions such as hydrogen environment.

Using electron channeling contrast imaging to inform and improve the growth of high-efficiency GaAs solar cells on nanopatterned GaAs substrates

Patterned substrates provide opportunities for reducing the cost of high-efficiency III-V devices by incorporating mechanically weak layers beneficial for substrate reuse (e.g. by spalling). In this work, the functionality of electron channeling contrast imaging (ECCI) as a tool to efficiently understand and mitigate defect formation is exemplified by developing a process in which high-quality III-V material can be grown on nanopatterned GaAs substrates. Reactive ion etching used in the patterning process was found to damage the GaAs substrate surface, leading to the formation of stacking faults in the epitaxial material as observed by ECCI. Etching the patterned substrates in a 1 NH4OH: 1 H2O2: 50 DI H2O solution for 10 s prior to growth removed the substrate surface damage and stacking faults were no longer present. Growth of solar cell device structures initially produced samples with many macroscale flaws creating shunts in the devices, which complicated the assessment of material quality by device measurements. However, ECCI revealed that the epitaxial material surrounding macroscale flaws was free from any crystallographic defects such as stacking faults and threading dislocations. With this knowledge, we focused on refining the patterning process to eliminate the macroscale flaws. Solar cells were then grown on the improved nanopatterned substrates and exhibited device structures with defect densities less than 5 × 105 cm−2 and average conversion efficiency of 24.8%, nearly identical to devices grown on unpatterned epi-ready substrates (25.0%).

Quantification and Characterization of Microdamage Resistance in Metals for Designing High-Strength Ductile Microstructures

ConspectusFractures in metallic materials such as ductile austenitic, ferritic, and dual-phase steels often occur after significant plastic deformation. The dislocation-driven plasticity enhances the microscopic stress concentration and induces vacancies (e.g., by the jogged screw dislocation motion), resulting in microstructure-scale cracks and voids at specific crystallographic planes or microstructure boundaries. (Hereafter, cracks and voids are referred to as damage.) In addition, plasticity plays important roles in microscopic damage growth in terms of damage tip blunting and coalescence. In fact, plasticity-related damage evolution in various fracture phenomena such as metal fatigue and hydrogen embrittlement still contains many uncertainties, particularly in high-strength materials containing fine and complex microstructures. Therefore, microscopy-based damage quantification and characterization are required for designing damage-resistant microstructures. In this context, because damage evolution is a multiscale phenomenon from atomistic to over millimeter scales, the target size of damage in the measurements is important to realize reliable analyses. The intrinsic origin of damage evolution is an atomistic/nanometer scale phenomenon, which requires transmission electron microscopy for its analysis. However, from mechanical viewpoints, revealing damage nucleation behavior is not necessary, because general mechanical analyses are performed for oversubmicrometer-sized damages that can be observed by optical microscopy and scanning electron microscopy. Hence, submicrometer/micrometer-sized damage is the major target in this damage quantification. As a post-mortem analysis method, the damage area fraction, the number of damages, the damage size, and the damage shape at various plastic strains can be quantified by observing micrometer-sized damages. In the case of monotonic tensile deformation, the number density of damages plotted against strain indicate the damage initiation probability. In addition, when damages stop growing, a strain range where the average damage size remains nearly constant appears. The strain range quantitatively indicates damage arrestability. For instance, dual-phase steel consisting of soft and hard body-centered cubic phases clearly shows three strain-dependent damage evolution regimes: (1) damage incubation regime (no damage appears), (2) damage arrest regime (damage initiates, but stops growing immediately), and (3) damage growth regime. Hydrogen uptake increases damage initiation probability and decreases damage arrestability. Specifically, a comparison of the quantified damage parameters between specimens with and without hydrogen revealed that large amounts of hydrogen in the dual-phase steel degrades the resistance to microscopic damage growth, which critically decreases the macroscopic ductility. In this case, the damage arrestability (damage growth resistance) is more important than crack initiation resistance, which is dependent on the dislocation-driven stress accommodation capability of soft microstructure components surrounding the crack initiation site. Thus, damage quantification enables the evaluation and classification of microscopic problems in plasticity-related fractures. After the classification of the problems, the underlying mechanisms for damage initiation and growth must be clarified separately. To reveal the mechanisms, full-field micrometer-scale strain mapping and site-specific dislocation characterization are required for damage initiation and growth. To fulfill these demands, new techniques were developed, such as micrometer-scale replica digital image correlation and dislocation-resolved in situ electron channeling contrast imaging. Based on the deformation/dislocation characterization results, metallurgical models of damage evolution can be developed, enabling to propose a mechanism-based microstructure-design strategy for creating damage-resistant high-strength materials.

Quantitative study of microstructural, textural and hardness evolution of high-purity Ti sheet during rolling from low to medium strains

Many Ti-base products are made of sheets/plates processed by rolling and strong crystallographic textures are often developed, especially for those with α-Ti as the major phase. Compared to intensive explorations of deformation behaviors of low alloyed Ti (including commercial-purity Ti) during rolling, much less has been made for high-purity Ti (HP-Ti), in spite of acknowledged important effects of some impurities on deformation modes of α-Ti. To clearly reveal microstructural, textural and hardness evolution of HP-Ti during rolling from low to medium strains, a typical HP-Ti sheet after 10–50% cold rolling was subjected to quantitative characterization by jointly using electron backscatter diffraction, electron channel contrast imaging, X-ray diffraction and hardness test. Results show that plastic deformation readily occurs through the active operation of both slip and twinning (mainly {112¯2}<112¯3¯> and {101¯2}<101¯1¯>) during 10–30% rolling. As a result of massive twinning, initial grain structures are markedly refined along with significant grain reorientation, leading to largely reduced textural intensity and the presence of new components. At higher strains (>30%), slip becomes the only deformation mode with new twins no longer appearing. In the 50%-rolled specimen, a bimodal basal texture similar to the initial one is formed along with a weak component of c//TD. The HP-Ti sheet is always hardened with increasing strains (from 120.8 ± 5.7 HV to 234.3 ± 5.8 HV). Quantitative analyses reveal that the grain refinement caused by dense twins at low strain (10% rolling) can lead to significant hardening contribution, which keeps relatively stable during subsequent rolling. Since slip can play a more important role with increasing strains, denser low angle boundaries are produced and gradually make larger contributions to hardening. As the rolling reduction increases to 50%, the contribution from low angle grain boundaries to hardness exceeds that from high angle grain boundaries (Hall-Petch hardening). The results documented in this work should not only be able to help clarify specific deformation mechanisms of HP-Ti, but also provide important implications for improving their properties.

Hydrogen‐assisted fatigue crack propagation behavior of equiatomic Co–Cr–Fe–Mn–Ni high‐entropy alloy

AbstractThe equiatomic Co–Cr–Fe–Mn–Ni high‐entropy alloy (HEA) shows well hydrogen embrittlement resistance under monotonic tensile load. However, the fracture behavior under cyclic load is still unclear. In this study, combining with the fracture features analysis by electron back‐scattered diffraction and electron channeling contrast imaging techniques, the hydrogen‐assisted fatigue crack propagation behavior of equiatomic Co–Cr–Fe–Mn–Ni HEA under in situ electrochemical hydrogen charging was investigated. The results suggest that the hydrogen had significant accelerating effects on the fatigue crack growth rate of Co–Cr–Fe–Mn–Ni HEA. Intergranular cracking with the formation of dislocation cells was observed at low stress intensity range (ΔK) area, while transgranular cracking with deformation twins was observed at a high ΔK area. The formation of these deformation features was assisted by hydrogen‐assisted dislocation emission.

Cold spray deposition characteristic and bonding of CrMnCoFeNi high entropy alloy

Particle bonding physics remains an important area of study in cold spray (CS) deposition. It has been argued that for pure metals, large interfacial strains in particles triggered either by adiabatic shear instability (ASI) or hydrostatic plasticity promote bonding while the bonding mechanisms of high entropy alloys (HEA) have not been explored. HEAs are an emerging class of materials that have superior work-hardening ability and are usually resistant to softening, a behavior that is different from pure elements and conventional alloys. In this study, equiatomic CrMnCoFeNi HEA feedstock powder is produced and CS experiments are conducted to explore the deformation evolution of these HEA particles and their bonding characteristics. The shear localization strain of the HEA is theoretically estimated and compared to conventional alloys and elements to explain the deposition behavior of the HEA. Results show that the particle impact morphology is strain rate dependent in addition to being material and microstructure dependent. Electron channelling contrast imaging reveals severe plastic deformation at the lower half of the particles due to the dynamic inertia effect. Deformation nano-twins with a thickness of about 200 nm were observed. HEA/HEA pair has a higher critical velocity compared to HEA/Nickel, HEA/Inconel625 and HEA/Stainless steel 304 pairs. This is attributed to the excellent strain hardening and intermediate softening of the HEA alloy used, which potentially postpones shear localization and impedes bonding, requiring a higher deposition velocity. Finally, it is shown that the ASI based criterion for bonding can predict the deposition mechanism of HEAs more inclusively.

(Invited) Nanoscale Investigation of Extended Defects in Wide Bandgap Semiconductors By Exploiting Phonon-Resonant Near-Field Interaction

The use of wide-bandgap semiconductor materials like SiC and GaN can dramatically improve the performance of electronic devices at high powers and temperatures. However, the propensity of extended defects in these materials does challenge their implementation in commercial electronic and optical applications. Spectroscopic and microscopic tools for identifying and characterizing these defects typically offer either spectroscopic or microscopic information exclusively and/or may be destructive. Here, we show how extended defects within 4H-SiC manifest in the nanoscale infrared phonon response probed by scattering-type scanning near-field optical microscopy (s-SNOM), a nondestructive method capable of simultaneously collecting topographic and spectroscopic information with frequency-independent nanoscale spatial resolution (≈20 nm).We correlate the s-SNOM response of various defects in 4H-SiC with UV-photoluminescence, secondary electron and electron channeling contrast imaging, and transmission electron microscopy. We identify evidence of step-bunching, recombination-induced stacking faults, and threading screw dislocations, and also demonstrate the interaction of surface phonon polaritons with extended defects. Phonon-enhanced infrared nanospectroscopy and spatial mapping via s-SNOM thus offer significant insights into extended defects within emerging semiconductor materials and devices. It thus serves as an important diagnostic tool to help advance material growth efforts for electronic, photonic, phononic, and quantum optical applications.[1] B. Hauer, C. E. Marvinney, et al. Advanced Functional Materials 30, 1907357 (2020).

GaAs on Si substrate with dislocation filter layers for wafer‐scale integration

GaAs on Si grown via metalorganic chemical vapor deposition is demonstrated using various Si substrate thicknesses and three types of dislocation filter layers (DFLs). The bowing was used to measure wafer-scale characteristics. The surface morphology and electron channeling contrast imaging (ECCI) were used to analyze the material quality of GaAs films. Only 3-μm bowing was observed using the 725-μm-thick Si substrate. The bowing shows similar levels among the samples with DFLs, indicating that the Si substrate thickness mostly determines the bowing. According to the surface morphology and ECCI results, the compressive strained indium gallium arsenide/GaAs DFLs show an atomically flat surface with a root mean square value of 1.288 nm and minimum threading dislocation density (TDD) value of 2.4 × 107 cm−2. For lattice-matched DFLs, the indium gallium phosphide/GaAs DFLs are more effective in reducing the TDD than aluminum gallium arsenide/GaAs DFLs. Finally, we found that the strained DFLs can block propagate TDD effectively. The strained DFLs on the 725-μm-thick Si substrate can be used for the large-scale integration of GaAs on Si with less bowing and low TDD.

Composition-dependent slip planarity in mechanically-stable face centered cubic complex concentrated alloys and its mechanical effects

In metallic materials, enhancing strain hardening capacity positively affects ductility, fracture toughness, ultimate tensile strength, and other properties. Thus, activating mechanically-induced martensitic transformation or twinning mechanisms has been a captivating goal in the design of steels, titanium alloys, cobalt alloys, complex concentrated alloys (CCAs) and others, through modifications to composition or thermo-mechanical processing. Here instead, we focus on the most basic strain hardening effect, arising from dislocation kinematics, and interactions. For this purpose, we designed two model face centered cubic (FCC) CCAs, Ni2CoCrFe and Ni2CoCrFeTi0.2Al0.1. Both alloys develop single, mechanically-stable, FCC phase microstructures upon processing. Mechanical tests of these alloys reveal that the Al and Ti addition enhances the strain hardening capacity, leading to significant increases in strength and ductility. Microstructure analyses based on electron channeling contrast imaging (ECCI), electron-backscatter diffraction (EBSD), and transmission electron microscopy (TEM) confirm the absence of mechanically-induced twinning and martensitic transformation effects, revealing instead a transition from wavy slip to planar slip. In-situ synchrotron XRD tensile tests are used to discuss the origin of the dislocation glide mode transition and the effects on strain hardening. Based on these analyses, the increased degree of short-range ordering (SRO), rather than the changes in stacking fault energy (SFE), is proposed as the main cause for this transition, and the corresponding effects on strain hardenability.

Investigation of Alloy‐Dependent Occurrence of Ferromagnetism in Carbon‐Expanded Austenitic Steel after Low‐Temperature Surface Hardening

Ferromagnetism is of increasing importance in the growing field of electromobility and data storage. In stable austenitic steels, the occurrence of ferromagnetism is not expected and would also interfere with many applications. However, ferromagnetism in austenitic stainless steels after low‐temperature nitriding has already been shown in the past. Herein, the presence of ferromagnetism in austenitic steels is discovered after low‐temperature carburization (Kolsterizing), which represents a novel and unique finding. A zone of expanded austenite is established on various austenitic stainless steels by low‐temperature carburization and the respective ferromagnetism is investigated in relation to the alloy composition. The ferromagnetism occurring is determined by means of a commercial magnetoinductive sensor (Feritscope). Ferromagnetic domains are visualized by magnetic force microscopy and a ferrofluid. X‐ray diffraction measurements indicate a clear difference in the lattice expansion of the different alloys. Furthermore, a different appearance of the magnetizable microstructure regions (magnetic domain structure) is detected depending on the grain orientation determined by electron backscatter diffraction (EBSD). Strongly pronounced magnetic domains show no linear lattice defects, whereas in small magnetizable areas linear lattice defects are detected by electron channeling contrast imaging and EBSD.

Monolithically integrated InGaAs/AlGaAs multiple quantum well photodetectors on 300 mm Si wafers

Near infrared light detection is fundamental for sensing in various application fields. In this paper, we detail the properties of InGaAs/AlGaAs multiple quantum well (MQW) photodetectors (PDs) monolithically integrated by direct epitaxy on 300 mm Si(001) substrates. A MQW high crystalline quality is achieved using 300 mm Ge/Si pseudo-substrates with a low threading dislocation density of 4 × 107 cm−2 from electron channeling contrast imaging measurements. The localized states in the MQW stack are investigated using temperature-dependent photoluminescence. Two non-radiative recombination channels are identified. The first one is due to delocalized excitons generated by potential’s fluctuations because of the InGaAs/AlGaAs interfacial roughness (with an activation energy below 4 meV). The second one is due to exciton quenching because of the presence of numerous threading dislocations. A low dark current density of 2.5 × 10−5 A/cm2 is measured for PDs on Ge/Si substrates, i.e., a value very close to that of the same PDs grown directly on GaAs(001) substrates. A responsivity of 36 mA/W is otherwise measured for the photodiode on Ge/Si at room temperature and at −2 V.

Magnetic properties and microstructural characterization of cold-rolled and annealed 317L austenitic stainless steel

The annealing behavior of cold-rolled 317L austenitic stainless steel was investigated. The material was rolled to a true strain (ε) of 2.04 and subjected to both stepwise and continuous annealing up to 1000 °C, the latter conducted in the presence of an external magnetic field. Electron backscatter diffraction, dilatometry, thermodynamic calculations, Vickers microhardness testing, and electron channeling contrast imaging were used to follow the microstructure evolution upon annealing. The microstructure of the cold-rolled steel has about 2.3% of strain-induced α′-martensite, 4% of delta ferrite, and austenite as the predominant phase. Eye-shaped deformation heterogeneities are also noticeable and contrast with the predominant lamellar structure typical of cold-rolled materials. Microstructure changes were followed by means of magnetic measurements, with emphasis on both Ms (saturation magnetization) and Hc (coercive field) parameters. Our findings confirm the occurrence of austenite reversion, decrease of delta ferrite, and massive sigma phase precipitation for the annealing temperature and time intervals herein investigated. At 800 °C the steel is almost fully recrystallized, except for the eye-shaped structures, in whose interior precipitation is much less intense. Precipitation of sigma phase occurs preferentially at the delta ferrite lamellae. Magnetization was able to capture the fragmentation of the ferromagnetic delta ferrite lamellae due to sigma phase precipitation and the changes associated with the decrease of delta ferrite and austenite reversion upon annealing.

Quantitative misfit dislocation characterization with electron channeling contrast imaging

Quantitative misfit dislocation characterization with electron channeling contrast imaging

Advances in heteroepitaxial integration of III-V and IV-VI semiconductors with electron channeling contrast imaging

Advances in heteroepitaxial integration of III-V and IV-VI semiconductors with electron channeling contrast imaging

Failure Analysis in FeCo Magnetic Alloys through Electron Channeling Contrast Imaging Defect Characterization

Failure Analysis in FeCo Magnetic Alloys through Electron Channeling Contrast Imaging Defect Characterization

Localized deformation inside the Lüders front of a medium manganese steel

Medium manganese steels are promising alloys for highly demanding sheet steel applications. However, cold-rolled intercritically annealed ultrafine-grained dual-phase medium manganese steels exhibit Lüders banding, which is detrimental for the mechanical properties and for the surface finish after forming. Therefore, it is essential to prevent the formation of Lüders bands. To achieve this, it is crucial to understand the underlying micromechanisms of their formation and propagation. While the nucleation of Lüders bands was studied before in these alloys, understanding of their propagation is lacking. Here, we use electron channelling contrast imaging, with a resolution of ~8 nm to examine the defect populations within and across the Lüders front. While macroscopic digital image correlation shows a uniform strain gradient across the Lüders front, quasi in-situ tensile tests in combination with microscopic electron channelling contrast imaging investigations reveal strain localized regions (termed plastic zones) within the Lüders front. These plastic zones are aligned with the macroscopic Lüders front and parallel in nature, with less deformed regions sandwiched between them. Additionally, it is observed that the size and distribution of the plastic zones are affected by the mechanical stability of austenite. We further demonstrate the role of strain partitioning between the austenite and ferrite (austenite being the softer phase) in the formation of the plastic zones.