Absorption‐induced enhancement of <scp>X</scp> ‐ray contrast by soft <scp>X</scp> ‐ray emissions

Contrast in atomically‐resolved EDX elemental mapping in real space or in reciprocal space channelling patterns, derived from characteristic X‐ray emissions excited by a given probe, is generated by the variation in emission rate as the incident wave function is scanned over the target atoms in a crystal. This is achieved either by raster scanning a focused coherent probe in real space at a fixed orientation (conventional EDX mapping), or by a systematic scan in angle of a collimated beam, the entire EDX spectrum being collected for each pixel (ALCHEMI). Whilst the signal is essentially governed by the probability density of the probe wave function on each atom site, delocalization of the primary ionization event may lead to a reduction in contrast since the electron wave function is effectively sampled over a finite region. The total ionization potential may be described in real space as a Lorentzian function of half‐width b , and the event becomes more localized with an increase in X‐ray energy. Thus coherent contrast provides a map in real or reciprocal space of the electron beam interaction with the ionization potential of a specific atomic species. However incoherent scattering by an absorptive potential leads to a progressive increase in a separate incoherent background component, this being associated with an EDX spectrum which is representative of the overall specimen composition. Generally speaking, coherent contrast is generated within the top 10 – 30 nm, and this initial contrast is progressively undermined by the incoherent background contribution in the form of an absorptive potential in atomic resolved maps or thermal diffuse scattering Kikuchi band contrast in channelling patterns [1]. The relative detected proportion of coherent compared with incoherent signal is altered by X‐ray absorption within the specimen. It is envisaged that strong absorption of soft X‐rays (described by a small mean free path λ) will enhance coherent contrast in both lattice image and channelling patterns compared to that derived from more energetic X‐rays. High energy X‐rays should have better contrast in the coherent signal due to increased localization, but this begins to be dominated by an incoherent signal that builds up with thickness. Softer X‐rays may have somewhat diminished coherent contrast due to greater intrinsic delocalization, but this is offset by a more strongly attenuated incoherent signal with increasing thickness. X‐ray channelling patterns near the orientation were obtained from GaAs using 300 keV electrons together with a custom‐built acquisition script run on an FEI Titan TEM. Fig. 1 shows PCA noise‐reduced patterns compared with simulations in which both delocalization and absorption are taken into account. Close inspection reveals greater overall dynamic contrast occurs for softer X‐rays. Experimental K/L ratio maps are shown in Fig. 2 (tilt 26° and take off angle 21°), being consistent with the calculation where the central As K/L contrast ratio starts to diminish with increasing thickness due to X‐ray absorption (λ K = 16 μm, λ L = 0.15 μm, ( b K = 0.024 Å, b L = 0.13 Å) . A similar but smaller effect occurs for the mapped Ga K/L ratio (λ K = 42 μm, λ L = 1.2 μm, b K = 0.027 Å, b L = 0.15 Å) since Ga L‐shell X‐rays are less strongly absorbed than the As L. Without X‐ray absorption, a central enhancement remains for all thicknesses, and the harder X‐rays retain more contrast. Fig. 3 shows the separate coherent and incoherent components together with the total contribution. Note the incoherent contrast is similar for all excitations. In conclusion, experimental observations are consistent with a model that predicts an increase in contrast from soft X‐rays compared with more energetic X‐rays, thus demonstrating that X‐ray absorption may play a greater role than delocalization in determining overall contrast. It is anticipated that X‐ray absorption, enhanced by grazing angle specimen‐detector geometry, may be useful in enhancing the coherent/incoherent scattering for soft X‐rays. Calculations have shown analogous effects occur in atomically‐resolved X‐ray STEM imaging [1].

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