Exploring transmission Kikuchi diffraction using a Timepix detector

Electron backscatter diffraction (EBSD) is a well-established scanning electron microscope (SEM)-based technique [1]. It allows the non-destructive mapping of the crystal structure, texture, crystal phase and strain with a spatial resolution of tens of nanometers. Conventionally this is performed by placing an electron sensitive screen, typically consisting of a phosphor screen combined with a charge coupled device (CCD) camera, in front of a specimen, usually tilted 70° to the normal of the exciting electron beam. Recently, a number of authors have shown that a significant increase in spatial resolution is achievable when Kikuchi diffraction patterns are acquired in transmission geometry; that is when diffraction patterns are generated by electrons transmitted through an electron-transparent, usually thinned, specimen. The resolution of this technique, called transmission Kikuchi diffraction (TKD), has been demonstrated to be better than 10 nm [2,3]. We have recently demonstrated the advantages of a direct electron detector, Timepix [4,5], for the acquisition of standard EBSD patterns [5]. In this article we will discuss the advantages of Timepix to perform TKD and for acquiring spot diffraction patterns and more generally for acquiring scanning transmission electron microscopy micrographs in the SEM. Particularly relevant for TKD, is its very compact size, which allows much more flexibility in the positioning of the detector in the SEM chamber. We will furthermore show recent results using Timepix as a virtual forward scatter detector, and will illustrate the information derivable on producing images through processing of data acquired from different areas of the detector. We will show results from samples ranging from gold nanoparticles to nitride semiconductor nanorods

Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black–white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated

Improving EBSD precision by orientation refinement with full pattern matching

We present a comparison of the precision of different approaches for orientation imaging using electron backscatter diffraction (EBSD) in the scanning electron microscope. We have used EBSD to image the internal structure of WC grains, which contain features due to dislocations and subgrains. We compare the conventional, Hough-transform based orientation results from the EBSD system software with results of a high-precision orientation refinement using simulated pattern matching at the full available detector resolution of 640 × 480 pixels. Electron channelling contrast imaging (ECCI) is used to verify the correspondence of qualitative ECCI features with the quantitative orientation data from pattern matching. For the investigated sample, this leads to an estimated pattern matching sensitivity of about 0.5 mrad (0.03°) and a spatial feature resolution of about 100 nm. In order to investigate the alternative approach of postprocessing noisy orientation data, we analyse the effects of two different types of orientation filters. Using reference features in the high-precision pattern matching results for comparison, we find that denoising of orientation data can reduce the spatial resolution, and can lead to the creation of orientation artefacts for crystallographic features near the spatial and orientational resolution limits of EBSD

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

Diffractive triangulation of radiative point sources

We describe a general method to determine the location of a point source of waves relative to a twodimensional single-crystalline active pixel detector. Based on the inherent structural sensitivity of crystalline sensor materials, characteristic detector diffraction patterns can be used to triangulate the location of a wave emitter. The principle described here can be applied to various types of waves, provided that the detector elements are suitably structured. As a prototypical practical application of the general detection principle, a digital hybrid pixel detector is used to localize a source of electrons for Kikuchi diffraction pattern measurements in the scanning electron microscope. This approach provides a promising alternative method to calibrate Kikuchi patterns for accurate measurements of microstructural crystal orientations, strains, and phase distributions

Electron channelling contrast imaging for III-nitride thin film structures

Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI; especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed

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

Microscopy of defects in semiconductors

In this chapter, the authors discuss microscopy techniques that can be useful in addressing defects in semiconductors. They focus on three main families: scanning probe microscopy, scanning electron microscopy and transmission electron microscopy. They first address the basic principles of the selected microscopy techniques In discussions of image formation, they elucidate the mechanisms by which defects are typically imaged in each technique. Then, in the latter part of the chapter, they describe some key examples of the application of microscopy to semiconductor materials, addressing both point and extended defects and both two-dimensional (2D) and three-dimensional (3D) materials

Dislocation contrast in electron channelling contrast images as projections of strain-like components

The forward scattering geometry in the scanning electron microscope enables the acquisition of electron channelling contrast imaging (ECCI) micrographs. These images contain diffraction information from the beam of electrons “channelling in” into the sample. Since small, localised strains strongly affect the electron diffraction, defects which introduce lattice displacement in the region of the surface the electron beam is interacting with will be revealed as district variation in backscattered electron intensity. By acquiring multiple images from the same area in different diffraction conditions and comparing them against modelled predictions of defect strain sampled by diffraction, it is possible to characterise these defects. Here we discuss the relation between the elastic strain introduced by a threading dislocation intersecting the surface and the contrast features observed in the electron channelling contrast image of that region. Preliminary simulated channelling contrast images are shown for dislocations with known line direction and Burgers vectors using a two-beam dynamical diffraction model. These are demonstrated to be in qualitative agreement with measured images of dislocated polar wurtzite GaN acquired with two different diffraction condition