Computer modelling for Fresnel contrast analysis

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Abstract
The analysis of the Fresnel contrast visible at interfaces seen edge-on in the electron microscope provides a useful method for determining the nature of the composition change at these interfaces, enabling, for example, the degree of interface diffuseness to be assessed with near atomic-spacing accuracy. The method relies on the comparison of the Fresnel contrast seen experimentally with that produced by computer simulations for different models of the variation in composition across the interface, the model being altered until the best fit is obtained. Different types of computer model can be used to simulate the Fresnel contrast, with the choice of model depending on the experimental conditions. In this paper we discuss the problems associated with the computer modelling of Fresnel contrast in order to establish the potentially very high accuracy which can be achieved by the use of the Fresnel method in measuring interface diffuseness, layer width and composition change; this also enables us to consider the types of system to which the method is best applied. Our discussion of inelastic scattering and the effects of atomic bonding in the computer models is of relevance to high-resolution image simulation as well as to the ‘lower’-resolution Fresnel analysis which is our main concern here.