High-resolution Z-contrast imaging in the STEM

Abstract

With a high-angle annular detector in a STEM, incoherent imaging can be realized at atomic resolution. This provides improved image resolution, from 0.66 CS¼λ¾ to 0.43 CS¼λ¾, based on the Scherzer definition of point-to-point resolution, although in both cases finer scale detail can be transmitted to the image at higher defocus values. Equally important, the incoherent mode provides a simply interpretable image; no contrast reversals occur with defocus or sample thickness, no Fresnel fringe effects are seen at interfaces and no speckle pattern is seen within an amorphous phase. In addition, the high-angle (Rutherford) scattering imparts a strong compositional sensitivity to the image through the practically Z2 dependence of the cross section.How these effects occur have been discussed in detail elsewhere. The three components of high-angle scattering for a zone axis orientation are the HOLZ reflections, the zero layer reflections, and the thermal diffuse (Rutherford scattering) component. The contribution of the HOLZ is simple to estimate experimentally in selected area diffraction, either from bright field line traces or from the high-angle channeling pattern, which is obtained from the annular detector (Fig. 1). A HOLZ reflection would be visible in the center of the channeling pattern only if its intensity was greater than the total diffuse scattering reaching the annular detector.