The HREM use of the FEGTEM for the study of interfaces

A comparison is made of current approaches to the characterisation of the abruptness of an interface based upon, on the one hand, attempts to derive the exit wavefunction from a through focal image series and, on the other, indirect methods which seek to quantify the delocalised contrast seen at such interfaces as a function of the defocus. It is demonstrated that the delocalised contrast due to the presence of a lateral discontinuity in the projected forward scattering potential is both highly sensitive to the form of the local changes in the scattering behaviour for images at large defoci and also then potentially of extremely high contrast under the high spatial coherence conditions characteristic of the use of a FEGTEM. Given the problems inherent in the reconstruction of the exit wave function it is thus argued that if the primary interest is in the characterisation of an interface then indirect approaches would appear to be currently more useful than those based on attempts to derive pictures from the amplitude and phase of the exit wavefunctions. We suggest that this will be the case whether these pictures be obtained by filtering non-linear contributions to the experimental images or by full recursive restorations which potentially retain these non-linear contributions.