Conditions and reasons for incoherent imaging in STEM

The origin of incoherent imaging in STEM has been analysed by investigating the effects of the detector geometry and of the thermal vibrations of the atoms on the image formation. The conditions for incoherent imaging are discussed. In this case the Fourier transforms of the intensities at the exit plane of the object and at the image plane are linearly related with each other, The corresponding transfer function coincides with the modulation transfer function for incoherent imaging in TEM. By analysing the properties of the degree of coherence, the reasons for the suppression of the interference terms are shown and detector arrangements are found which yield largely incoherent images. The validity of the semianalytical results for thin objects are also confirmed numerically for thick objects by means of a modified multislice algorithm. With increasing object thickness the phonon scattered electrons dominate the image intensity. Detector arrangements were found for which the elastic part of the image shows contrast reversal. The dependence of the Z-contrast on the geometry of the annular detector and on the atomic number Z is investigated in detail.