Contributions to the contrast in experimental high-angle annular dark-field images

This paper reports on a study of the contributions to the image contrast of high-angle annular dark field (HAADF) images acquired in scanning transmission electron microscopy. Experimental HAADF images were obtained from a model system consisting of an epitaxial perovskite PbTiO3 film grown on a SrTiO3 single crystal. This sample allowed for the study of the intensities of a wide range of atomic numbers. The main objective of the paper was to quantify the influence of TEM foil thickness on the image contrast, but the effects of the annular detector inner angle and the probe forming lens focus were also studied. Sample thicknesses ranging from approximately 10 nm to more than 400 nm were investigated. The image contrast was relatively insensitive to changes in inner angle. The main impact of sample thickness was a rapid increase in a background intensity that contributed equally to the intensities of the atomic columns and the channels between them. The background intensity and its increase with thickness reflected the average atomic number of the crystal. Subtraction of the background intensity allowed for a quantitative interpretation of image contrast in terms of atomic numbers and comparison with multislice image simulations. The consequences for the analysis of interfaces in terms of atom column occupancies are discussed.