edfas.org 13 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 3 the ability to map magnetic domains,[6] serving to highlight the promise of the technique rather than ushering in widespread adoption. The implementation of 4D-STEM relies on the availability of detectors with low noise and high readout speeds, single electron sensitivity in addition to adequate radiation hardness. Early attempts at developing or repurposing detectors with these characteristics were met with varying degrees of success.[7-12] However, it was not until the mid2010s when detectors targeting 4D-STEM applications became more common and the number of commercial products in the market has increased steadily since then. A more in-depth discussion on detector technology and development is beyond the focus of this article but can be found elsewhere.[13] The remainder of this article will introduce various 4D-STEM methods related to imaging, Fig. 1 A schematic of scanning transmission electron microscopy including both a single channel and a pixelated detector, where α is the convergence semi-angle, 2θB is the Bragg angle of the diffracted beam labeled hkl (a). A colorized electron diffraction pattern where different features can be observed, including Bragg reflections, Kikuchi bands, and a diffuse background (b). An example 4D-STEM dataset with 578 individual diffraction patterns from GaN [12 _ 10]. The region of GaN, indicated by the ball model where Ga is green and N is yellow, was sampled in a 32 by 18 grid where each cross indicates a point in the scan (c). The diffraction patterns were simulated using the multislice method as implemented in the software program Dr. Probe.[56] (a) as the mid-1980s,[1] where two-dimensional diffraction patterns were recorded along a one-dimensional scan. 4D-STEM experiments were not far behind, with possibly the first demonstration being reported in 1989.[2] Initial experimentation was motivated by attempts to reconstruct real-space images from diffraction data and potentially surpass the image resolution of the microscope imposed by the lens aberrations.[3,4] These efforts culminated in a reconstructed image with a point resolution that far exceeded the information limit of the microscope,[5] and while incredibly impressive, these experiments required herculean efforts. The stability of the microscopes, detector technologies, and computing resources available at that time were inadequate to make this type of measurement practical. As a result, early 4D-STEM measurements amounted to isolated experiments, such as demonstrating (b) (c)
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