Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications

We analyze the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolor orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modeling of the energy and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channeling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations

Cross-correlation based high resolution electron backscatter diffraction and electron channelling contrast imaging for strain mapping and dislocation distributions in InAlN thin films

We describe the development of cross-correlation based high resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging (ECCI), in the scanning electron microscope (SEM), to quantitatively map the strain variation and lattice rotation and determine the density and identify dislocations in nitride semiconductor thin films. These techniques can provide quantitative, rapid, non-destructive analysis of the structural properties of materials with a spatial resolution of order of tens of nanometers. HR-EBSD has a sensitivity to changes of strain and rotation of the order of 10−4 and 0.01° respectively, while ECCI can be used to image single dislocations up to a dislocation density of order 1010 cm−2. In the present work, we report the application of the cross-correlation based HR-EBSD approach to determine the tilt, twist, elastic strain and the distribution and type of threading dislocations in InAlN/AlN/GaN high electron mobility transistor (HEMT) structures grown on two different substrates, namely SiC and sapphire. We describe our procedure to estimate the distribution of geometrically necessary dislocations (GND) based on Nye-Kroner analysis and compare them with the direct imaging of threading dislocations (TDs) by ECCI. Combining data from HR-EBSD and ECCI observations allowed the densities of pure edge, mixed and pure screw threading dislocations to be fully separated

Multicharacterization approach for studying InAl(Ga)N/Al(Ga)N/GaN heterostructures for high electron mobility transistors

We report on our multi–pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3μm)/AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X–ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson–Schrödinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2–DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance

Surface evolution of lithium titanate upon electrochemical cycling using a combination of surface specific characterization techniques

Among the electrodes for Li‐ion batteries, Li4Ti5O12 (LTO) stands out as anode owing to its stability and safety, in part ascribed to its low surface reactivity. However, the overlayer formation on the LTO surface upon electrochemical cycling is reported in recent years; a rough surface layer of electrochemically inactive α‐Li2TiO3 on top of the LTO (111) surface is suggested on the grounds of scanning probe techniques and theoretical ab initio calculations which would negatively strike on the battery performance. Hence the investigation of the LTO surface evolution is key to achieve more stable and safer Li‐ion batteries. LTO (111) thin film electrodes are used as model system where a variety of surface specific characterization techniques are applied to unveil the surface behavior of LTO in Li‐ion batteries. In contrast with previous studies, with the help of high‐resolution transmission electron microscopy and synchrotron‐based surface X‐ray diffraction, α‐Li2TiO3 is found to be a surface preparation product. Of special importance is the use of high‐resolution electron backscatter diffraction to report an increase on the LTO surface strain upon electrochemical cycling which can have a critical effect in long cycling performance of LTO that is always considered a zero‐strain material

Characterization of elastic strain field and geometrically necessary dislocation distribution in stress corrosion cracking of 316 stainless steels by transmission Kikuchi diffraction

Stainless steel alloys such as SUS 316 are widely used in nuclear power plants because of their excellent performance in high-temperature and corrosive environments. In this work, a stress corrosion crack from a sample tested under simulated primary water from a pressurized water reactor has been characterized

Applications of multivariate statistical methods and simulation libraries to analysis of electron backscatter diffraction and transmission Kikuchi diffraction datasets

Multivariate statistical methods are widely used throughout the sciences, including microscopy, however, their utilisation for analysis of electron backscatter diffraction (EBSD) data has not been adequately explored. The basic aim of most EBSD analysis is to segment the spatial domain to reveal and quantify the microstructure, and links this to knowledge of the crystallography (eg crystal phase, orientation) within each segmented region. Two analysis strategies have been explored; principal component analysis (PCA) and k-means clustering. The intensity at individual (binned) pixels on the detector were used as the variables defining the multidimensional space in which each pattern in the map generates a single discrete point. PCA analysis alone did not work well but rotating factors to the VARIMAX solution did. K-means clustering also successfully segmented the data but was computational more expensive. The characteristic patterns produced by either VARIMAX or k-means clustering enhance weak patterns, remove pattern overlap, and allow subtle effects from polarity to be distinguished. Combining multivariate statistical analysis (MSA) approaches with template matching to simulation libraries can significantly reduce computational demand as the number of patterns to be matched is drastically reduced. Both template matching and MSA approaches may augment existing analysis methods but will not replace them in the majority of applications

Corrigendum to “A mechanistic study of the temperature dependence of the stress corrosion crack growth rate in SUS316 stainless steels exposed to pressurized water reactor primary water” [Acta Mater. 114 (2016) 15–24]

The authors regret that one of the main contributors to this paper was unintentionally omitted from the author list in the final version of the manuscript. The complete author list should read: Martina Meisnara, Arantxa Vilalta-Clementea, Michael Moodya, Angus J. Wilkinsona, Koji Ariokab, Sergio Lozano-Pereza,∗ The authors would like to apologise for any inconvenience caused