Z-contrast Imaging in an Aberration-corrected Scanning Transmission Electron Microscope

Abstract

We show that in the limit of a large objective (probe-forming) aperture, relevant to a spherical aberration corrected microscope, the Z-contrast image of a zone-axis crystal becomes an image of the 1s Bloch states. The limiting resolution is therefore the width of the Bloch states, which may be greater than that of the free probe. Nevertheless, enormous gains in image quality are expected from the improved contrast and signal-to-noise ratio. We present an analytical channeling model for the thickness dependence of the Z-contrast image in a zone-axis crystal, and show that, at large thicknesses, columnar intensities become proportional to the mean square atomic number, Z2.

Column ratio mapping: A processing technique for atomic resolution high-angle annular dark-field (HAADF) images

“…For instance, the 1.4Å dumbbell spacing between Ga and As atomic planes along the [001] direction in GaAs (a zinc-blend structure) can be easily resolved in high angle annular dark field (HAADF) images from SuperSTEM 1. Such images are commonly referred to as atomic number (Z) contrast images since the recorded image intensity varies approximately with the square of the atomic number of the atomic columns present in the specimen due to Rutherford-like scattering [8][9][10][11][12]. Hence, columns with different mean Z 2 can be readily distinguished.…”

Synthesis, structure and properties of new functional oxynitride ceramics

“…In HAADF-STEM images, signal intensity is known to be roughly proportional to the square of the atomic number Z. 66) The bright spots in the image correspond to gallium ions. ABAB and ABCABC stacking sequences of gallium ions correspond to wurtzite and zinc-blende lattices, respectively.…”

Experimental evaluation of interfaces using atomic-resolution high angle annular dark field (HAADF) imaging

“…In order to investigate the interfacial sharpness of semiconductors (and other materials), high angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM) is frequently employed [6][7][8][9]. The underlying theory of HAADF imaging has been discussed by many authors [10][11][12][13][14][15][16]. In essence, this technique relies on the application of an annular detector with a large inner angle (>50mrad at 100kV accelerating voltage) in order to collect electrons that have undergone high angle elastic or thermal diffuse scattering.…”

“…In essence, this technique relies on the application of an annular detector with a large inner angle (>50mrad at 100kV accelerating voltage) in order to collect electrons that have undergone high angle elastic or thermal diffuse scattering. The collection of, in effect, Rutherford-like scattering results in the atomic number (Z) sensitivity that is typically associated with HAADF images [10][11][12][13][14][15][16]. In a simple qualitative model of image contrast, high-spatial resolution information in HAADF images is associated with atomic column intensities that sit on top of a background signal [17][18][19].…”