A Comparison of the Effects of Systematic Reflections on Extinction Contours in Bright‐ and Dark‐Field Images of Wedge Crystals

Foil thickness measurements are necessary for determining crystal volumes, which in turn are needed to calculate defect densities for dislocations or point defect clusters or for determining interparticle spacings. The most generally applied technique for obtaining the foil thickness is the trace method wherein the trace of a known plane (slip trace, stacking fault, grain boundary, etc.) must be determined, its projected width on a micrograph measured, the precise crystal orientation calculated from a diffraction pattern and the thickness calculated from simple trigonometric relationships. If no traces are obtainable on an image, an alternate method uses extinction contours: a perfect wedge-shaped crystal is set at the Bragg angle, and the number of white fringes from the edge of the foil in a bright-field micrograph (or dark fringes in a dark-field micrograph) counted and multiplied by the extinction distance (which must be accurately known) for the particular reflection involved.

The indexing of Precession Electron Diffraction (PED) patterns in TEMs for crystals orientation and phase determination as operated by the ACOM‐TEM technique [1] tends to become a standard procedure. However, analyzing transmitted signals requires to deal with significant effects related to the lamella thickness. Indexing limitations emerge as soon as grain size is smaller than the sample thickness. In contrast with TKD patterns [2], information in PED patterns comes from all of the overlapping grains crossed by the electron beam. Analyzing such a mixture of Bragg reflections prevents the safe recognition of the orientations and phases and leads to two specific outcomes. First, the grains appearing in the resulting maps are not necessarily located on a same exact layer of the sample thickness. Second, the probability of mis‐indexing is increased as reflections from every crystal may be taken for template matching. While it remains unclear which of the overlapping crystals is selected by template matching, it is necessary to understand if and in what means the Bragg spots number and related intensity of each crystal can differ from each other in the acquired patterns. In the present work, the influence of volume fraction and arrangement of grains with respect to the illumination direction are examined. A sample composed of two overlaid copper plates was considered for the present purpose. ACOM‐TEM characterizations were realized on a planar section of the stacked plates for different lamella orientations: at zero tilt, zero tilt after a 180° flip of the grid in the sample holder, and a 15° tilt. Seven cross sections were then cut to determine the respective microstructures and thicknesses of the superimposed plates. Using TEM images of the cross views, the overall thickness of the stacked plates was found to evolve from 330 nm to 470 nm from one side to the other, with one plate being 1.4 to 2 times thicker than the other. The crystallographic orientations were determined using the NanoMEGAS ASTAR TM system implemented on a FEI Tecnai G2 F20 S‐Twin FEG (S)TEM operating at 200 keV. A precession angle of 0.5° was systematically applied to a quasi‐parallel probe of 4 nm at HMFW and 0.4 mrad semi‐angle of convergence. TEM lamellae were prepared using a FEI HELIOS FIB. No significant differences are observed when the orientation maps related to the non‐tilted sample and its 180° flip are compared (Fig. 1c‐d). In other terms, the intensity distribution of Bragg reflections in diffraction patterns is not governed by the illumination direction. The comparison between the planar orientation maps and the cross cuts (Fig. 2) shows that the detected grains are mostly related to the thickest plate. At the light of this, it seems reasonable to expect the Bragg reflections related to the thickest plate to be the most intense and, consequently, the ones mainly detected in the acquired diffraction patterns. Nevertheless, a few grains related to the thinnest plate are indexed in the planar cuts. This means that the pattern selection is not solely sensitive to the volume fraction of the diffracting crystals. The last finding is confirmed with the sample tilted at 15°. With such tilt, the number of Bragg spots and their related intensities vary as different crystal planes are excited by the electron beam. It can be seen in Fig. 1b that some variations are detected in the grains orientation. The main conclusion of this study is that, although volume fraction of each plate is here the dominant factor that determines the template matching orientation selection, the correlation index appears to be also dependent on crystals orientation and potentially related dynamical effects. More details on the effects of volume fraction with respect to crystal favorable orientations will be discussed.

The dynamical theory of electron diffraction is applied to the interpretation of electron micro­ scopic images of lattice planes of plate- and wedge-shaped crystals. The wave functions and corresponding intensities predicting interference fringes on the exit surface of a crystal are derived. It is shown in both cases that the fringes are composed of parallel lines and the spacing of the fringes at the exact Bragg angle coincides with that of the original lattice but the positions of the lines do not coincide with those of potential maxima in the crystal, i.e. intensity profiles of the fringes do not represent the variation of mass-thickness in the crystal. The intensity profiles and the spacings of the fringes vary with the thickness of crystal and the deviation from the Bragg angle. The fringes from a bent plate-shaped crystal, which are formed on the extinction contour bands, show the same spacing as that of the crystal lattice along the centre of the contour but they have an increased or decreased spacing near the edge of the contour. The fringes which are formed on the subsidiary extinction contour also show spacing anomaly; they are shifted by half the corresponding amount for the principal contour. The spacing of the fringes of a wedge-shaped crystal coincides with that of the original lattice at the exact Bragg angle, but the contrast of the lines reverses wherever the thickness of the crystal increases by an amount of XE/2V g (A, wave length; E , accelerating potential; V g , Fourier coefficient of inner potential of the crystal). For deviation from the Bragg angle, the spacing of the fringes, in general, does not coincide with that of the original lattice and, moreover, the contrast of the lines reverses wherever the thickness of the crystal increases by an amount of The anomalies of spacing and reversal of contrast which are expected from the present theory were observed in the electron microscopic images of metal-phthalocyanine and sodium faujasite crystals respectively. The effects of absorption by the crystal and divergence of illumination on the contrast of the image are discussed and the possibility of obtaining two-dimensional projections of the atomic arrangement in a crystal by using electron microscopic images is also discussed.

SEM can be used to characterize the crystal structure at smooth surfaces, e.g. by mapping of electron channeling pattern (ECP). Layers of GaN grown on foreign substrates usually include a huge amount of threading dislocations (TDs). ECP are also used to align a sample in specific diffraction conditions of the crystal structure for evaluating the density of TDs and its type by electron channeling contrast imaging (ECCI) 1 . Beside columnar rods 2,3 also elongated µm‐structures like fins 4 with high aspect ratios are supposed to have substantial advantages over conventional planar optoelectronic and sensing devices. Thus the synthesis of such 3D‐structures, in particular by bottom up growth using molecular beam epitaxy and metalorganic vapor phase epitaxy (MOVPE) on patterned substrates, is under investigation ‐ requesting methods for characterizing local properties of the crystal material. We present results obtained with an FE‐SEM which is equipped with secondary electron (SE), In‐Beam SE, low‐kV backscattered electron (BSE), electron beam induced current (EBIC) and monochromatic CL detection as well as a piezo controlled manipulator setup, c.f. Figure 1. Simultaneous usage of all available detectors and the manipulator is possible, only the BSE and optical detection are physically hindered by another. A modified parabolic collection mirror enables measuring luminescence from planar samples in a tilted view up to 30°, with respect to the large chamber this enables also a nondestructive investigation of full 4''‐wafers. The electron optics (EO) of this FE‐SEM is also capable of rocking the electron beam on a small area, e.g. rocking in a cone of ±12° on an area of about 15 µm in diameter. We will present how the rocking alignment can be adjusted and evaluated by using samples with dedicated contrast structures. The EO tilt can also be used to image the sample by the SEM from a certain direction without affecting the tip contact by stage movement. This enables a fine adjustment of the diffraction conditions and subsequent ECCI images for evaluating the type of defects. A subsequent scanning of the sample from different incident directions enables topography reconstruction and generates a three dimensional impression, e.g. by a stereographic image. By switching from the BSE detector to the mirror for light collection also a correlative analysis of ECCI with EBIC and CL can be performed at the same diffraction condition, see Figure 2. Although having a certain topography contrast also 3D‐structures with dimensions of a few µm can be analyzed regarding their crystal structure and orientation using ECP. As CL is quite sensitive also to intrinsic and extrinsic point defects, such correlative images obtained on sidewall and cleavage edges of fin‐GaN structures will give valuable insights for discussion of defect mechanism and optical properties. As the resolution of channeling contrast (e.g. ECCI) and scattering volume (e.g. CL) versus the beam energy are different to another, a correlation of images obtained on the same sample area using different energies improves the identification of individual features. This demonstrates further options for investigating the material quality of 3D structures.

The resolution limit of Orientation Imaging Microscopy in the Scanning Electron Microscope is between 20 nA and 80 nA depending on the basic resolution/beam current performance of the SEM, the sample atomic number and the level of residual strain within it. The newer technique of orientation imaging in the transmission electron microscope, TEM, improves on this resolution limit by a factor of five to ten. The new technique is based on a novel procedure for determining the crystallography of separate small volumes in the sample by examination of a large series of dark field images. Each image is recorded for a different diffraction condition. This is achieved by using a computer to direct the electron beam onto the same area of the sample so that it covers all directions within a cone of semi-apex angle 3 degrees. Analysis of the intensity of the same point in each of the dark field images permits reconstruction of a diffraction pattern for that point providing the data to calculate its crystal orientation. The process is repeated for each point in the image. The Orientation Image Micrograph is constructed from the orientations so determined. The technique is shown to be capable of producing orientation micrographs of high spatial resolution for unstrained samples. For highly strained samples difficulties are encountered in accurately indexing the complicated diffraction patterns that are observed. Methods to improve the indexing procedures involve determining the sub-cell structure first from a comparison of patterns from adjacent pixels and then summing all patterns belonging to a single sub-cell. The resultant improvement in pattern quality permits more reliable determination of orientation. Examples of this procedure are taken from studies of deformed aluminum.

Electron diffraction contrast effects which arise at III-V compound heterostructure boundaries with small mismatches due to small differences in the diffraction conditions on either side of the boundary are discussed in detail. These effects can be characterized by a change in the deviation parameter s and give rise to δ fringes when the boundary is tilted with respect to the electron beam and also to a displacement of equal thickness fringes. Observations of the latter phenomenon require a well defined sample geometry and layers of known composition. δ fringe profiles are evaluated by comparison with theoretical profiles computed on the basis of equations from the two-beam dynamical theory for superposed crystals. The mismatch value extracted by this method agrees with the value expected for a GaAlAs/GaAs boundary provided that a relative change in crystal orientation at the boundary is allowed for rather than a simple tetragonal distortion perpendicular to the interface. The case of a GaInAsP/I...

The dislocation image has a line or dotted feature, which depends essentially upon the reflecting condition. This dependency has been clarified the dark field image method and also the method of small variation of the incident angle of electron beam, as follows:– Line images of dislocations: 1) Generally the contrast of dislocation image near an extinction contour is mainly due to the reflection of the same index as that of the contour. 2) The dislocation image, usually appears as a single line, sometimes as a double image, when it is due to one reflection. 3) On the contrary, sometimes the image looks like a single line even when it is due to multiple reflections. 4) The image having black contrast is generally accompanied white side-lines, and when the latter predominates, the image is observed as a white dislocation image. 5) Near the intersection of dislocations of two families, there are found some variations in the image features, which may be not only due to the interaction between strain fields of...

The crystal orientation effect on the fretting fatigue behavior in Ni-based single crystal superalloys (NBSX) was studied in this paper using crystal plasticity finite element (CPFE) simulation. The crystal plasticity constitutive model considering back stress was applied in the CPFE simulation, and four different crystal orientations were calculated to study the effect of crystal orientation. The CPFE simulation results showed significant differences between the four crystal orientations. The Mises stress and the cumulative shear strain under different crystal orientations was obtained. The change of stress and strain with time at different locations showed obvious difference under four crystal orientations. The simulation results showed obvious stress concentration near the contact edge, but the peak locations of cumulative shear strain would change under different crystal orientations.KeywordsFretting fatigueSuperalloyCrystal orientationCPFE simulation

Femtosecond Laser-Induced Formation of Surface Structures on Silicon and Glassy Carbon Surfaces

Biomineralization is one of the key processes that is notably affected in marine calcifiers such as oysters under ocean acidification (OA). Understanding molecular changes in the biomineralization process under OA and its heritability, therefore, is key to developing conservation strategies for protecting ecologically and economically important oyster species. To do this, in this study, we have explicitly chosen the tissue involved in biomineralization (mantle) of an estuarine commercial oyster species, Crassostrea hongkongensis. The primary aim of this study is to understand the influence of DNA methylation over gene expression of mantle tissue under decreased ~pH 7.4, a proxy of OA, and to extrapolate if these molecular changes can be observed in the product of biomineralization-the shell. We grew early juvenile C. hongkongensis, under decreased ~pH 7.4 and control ~pH 8.0 over 4.5months and studied OA-induced DNA methylation and gene expression patterns along with shell properties such as microstructure, crystal orientation and hardness. The population of oysters used in this study was found to be moderately resilient to OA at the end of the experiment. The expression of key biomineralization-related genes such as carbonic anhydrase and alkaline phosphatase remained unaffected; thus, the mechanical properties of the shell (shell growth rate, hardness and crystal orientation) were also maintained without any significant difference between control and OA conditions with signs of severe dissolution. In addition, this study makes three major conclusions: (1) higher expression of Ca2+ binding/signalling-related genes in the mantle plays a key role in maintaining biomineralization under OA; (2) DNA methylation changes occur in response to OA; however, these methylation changes do not directly control gene expression; and (3) OA would be more of a 'dissolution problem' rather than a 'biomineralization problem' for resilient species that maintain calcification rate with normal shell growth and mechanical properties.

Copper(I) thiocyanate (CuSCN) is known as a wide bandgap p-type semiconductor and recently demonstrated its high ability as a hole-transporting material in thin film devices such as dye-sensitized and perovskite solar cells [1]. However, in fact little is known for its intrinsic physical properties such as bandgap, band positions, optical transparency, carrier density and mobility.We have established methods to electrodeposit well-crystallized CuSCN thin films in various forms. Although the electrochemistry is fairly simple as limited by diffusion of 1 : 1 complex between Cu2+ and SCN- ions ([Cu(SCN)]+), the [Cu2+] : [SCN-] ratio, its absolute concentration and solvent can significantly alter the morphology and crystal orientation of resulting CuSCN [2]. Hybridization with various cationic organic dyes has also been achieved to furnish nanostructures and even transition from rhombohedral β to orthorhombic α form [3]. These unique features of the electrodeposition technique let us anticipate possibilities to tailor-tune physical properties of CuSCN to match the demands for the device applications. Moreover, electrodeposited CuSCN doesn’t hinder its use in flexible electronics unlike many other inorganic materials, since the process is done at room temperature.In this study, we have carried out electrodeposition of CuSCN to vary its morphology and crystal orientation by tuning the bath composition and studied their band structure to explore the room for tuning its physical properties.Morphologies of CuSCN thin films electrodeposited from stoichiometric (REF), Cu-rich and SCN-rich baths are compared in Figs. 1 a-c. While the REF sample has an open structure made of relatively large bulky particles, the Cu-rich sample is dense, made of tiny grains. XRD patterns have found almost random crystal orientation for the former and a high degree of preference of the latter to orient the c-axis of β-CuSCN perpendicular to the substrate. On the other hand, the one from the SCN-rich bath is made of elongated platelets (Fig. 1 b) and strongly oriented to lay down the c-axis in parallel with the substrate.As expected from the morphology, the Cu-rich film is highly transparent, whereas the other two are opaque due to light scattering (Table 1). Tauc plot for indirect transition indicated a systematic increase of bandgap from 3.58 to 3.64 eV on increasing the Cu2+ content. More significant difference was found for their work function (WF) measured by photoelectron yield spectroscopy (PYS). The threshold energy moved downwards from 5.31 to 5.66 eV vs. VAC from Cu-rich to SCN-rich film. The result indicates a high level of p-type doping in the presence of excess SCN-, probably due to increased concentration of Cu2+ as stabilized by SCN- bound to it.Enhanced photoluminescence has also been found for the electrodeposited CuSCN thin film especially for the SCN-rich sample, as compared to commercial CuSCN powder. Thus, electrodeposition has turned out to allow fine-tuning of physical properties of CuSCN for device applications.[1] Vinod E. Madhavan et al., ACS Energy Lett. 2016, 1, 1112−1117.[2] Lina Sun et al., Physics Procedia, 2011, 14, 12-24.[3] Yuki Tsuda et al., Chem., 2017, 148, 845-854. Figure 1

The present work investigates the dominant mechanisms in the plasticity of nano-sized fcc metallic samples. Molecular dynamics simulations of nanopillar compression show that plasticity always starts with the nucleation of dislocations at the free surface, and the crystal orientation affects the subsequent microstructural evolution. The Schmid factor of leading and trailing partials plays a decisive role in leading to the twinning, or slip deformation. A significant difference is observed in the strength of pillars of the same size with different orientations. The power-law equation exponent is completely dependent on the crystal orientations, and a weak or no size effect is observed in the compression of [100]- and [110]-oriented nanopillars. The observed orientation based behaviour decreases by confining the free surface.

In order to accomplish high performance and miniaturization, large size chip assembly and 3-dimensional packages has been required. Stress in ultra-low k (ULK) layer and high warpage during flip chip bonding process are the most challenging issues. For this reason, a low temperature and low stress bonding process has been required. In this report, we will investigate the strain change at bump connection area during thermal compression bonding (TCB) process and compared with that of Mass reflow process using a large size chip (20x20 mm). We will also investigate the difference of strain change between two kinds of solder, Sn3.0wt%Ag0.5wt%Cu (SAC) and Sn57wt%Bi (SnBi) solder. Microstructure and crystal orientation analyses of the connection bump were observed by Electron backscattered diffraction (EBSD). We prepared two types of organic laminate, which had different shrinkage factor 1.000 and 0.999. With SAC solder, both cases of TCB and Mass reflow, the Grain Reference Orientation Deviation (GROD) value of the edge bump was higher than that of the center bump. The size of crystal grain of Sn with TCB was smaller compared with that with Mass reflow process. Using the organic laminate with 0.999 shrinkage factor, the GROD value of the edge bump showed nearly the same value of another location. By optimizing the organic laminate shrinkage factor, TCB process is expected to achieve high reliability of the micro joining compared with Mass reflow process. On the other hand, with Sn57Bi solder, in both cases of TCB and Mass reflow, the size of crystal grain of Sn was smaller compared with that of SAC. And significant difference in crystal orientation and in micro structure were not observed between TCB and Mass reflow. Sn-Bi solder joining are strong candidate materials for stress reduction for large size chip application and 3D packages.

Comparing empirical interatomic potentials to modeling silicon surface stress

Theory and application of electron channelling contrast imaging under controlled diffraction conditions