Measuring the contrast in annular dark field STEM images as a function of camera length

Image contrast in annular dark-field (ADF) imaging in scanning transmission electron microscopy (STEM) is studied as function of the inner collection angle of the annular detector. The smaller the camera length is, the larger is the effective inner detection angle. As a result, the intensity recorded drops dramatically but the contrast increases with detection angle. For very large angles (high-angle ADF), Rutherford scattering by the atomic nuclei is the dominant scattering mechanism. The intensity is then simply proportional to the product of thickness (=number of scattering atoms) and the square of the atomic number (=Z-contrast), however, sufficiently high angles are experimentally difficult to obtain. By extrapolating the dependence of the image contrast as a function of camera length (=inverse of the width of the detector hole in the diffraction plane) to zero camera length (=infinite scattering angle) we can quantify the chemical composition in systems like InGaAs/GaAs thin film heterostructures where the concentration of only one element (here: indium) needs to be considered.