High-resolution annular dark-field STEM imaging

Abstract

Annular dark field STEM imaging has become an important tool as both a high resolution imaging technique showing Z-contrast, and as a way to locate the probe on the specimen while simultaneously recording other data such as electron energy loss spectra. It is intrinsically quantitative since image data can be recorded directly from linear detectors into digital memory Experiments on simple samples,and a multislice simulation approach backed by experimental evidence has been used to explore the image dependence on experimental factors such as inner detector angles and other elements in the imagingprocess Earlier work demonstrated marked thickness dependence in high Z specimens.In annular dark field imaging, the resolvable spatial resolution is roughly equal to the size of the electron probe One important factor in forming a small probe, and therefore in getting high resolution images, is an electron gun with a small virtual source size. An example is shown in Figure 1, which plots the relativestrengths of the 2.9Å and 2.1 Å fringes taken from experimental images of InP (100) What is seen is that the 2.1Å fringe intensity, at the extreme limits of resolution for this probe (approx 2.2Å in size), changes greatly in comparison with the more easily resolvable 2.9Å fringe as the source demagnification, in effect the source size, is changed.