CONDITIONS AND REASONS FOR INCOHERENT IMAGING IN STEM

The origin of incoherent imaging in STEM has been analysed by investig ating the effects of the detector geometry and of the thermal vibratio ns of the atoms on the image formation. The conditions for incoherent imaging are discussed. In this case the Fourier transforms of the inte nsities at the exit plane of the object and at the image plane are lin early related with each other, The corresponding transfer function coi ncides with the modulation transfer function for incoherent imaging in TEM. By analysing the properties of the degree of coherence, the reas ons for the suppression of the interference terms are shown and detect or arrangements are found which yield largely incoherent images. The v alidity of the semianalytical results for thin objects are also confir med numerically for thick objects by means of a modified multislice al gorithm. With increasing object thickness the phonon scattered electro ns dominate the image intensity. Detector arrangements were found for which the elastic part of the image shows contrast reversal. The depen dence of the Z-contrast on the geometry of the annular detector and on the atomic number Z is investigated in detail.