No abstract

We discuss an intuitive approach to interpreting mirror electron microscopy (MEM) images, whereby image contrast is primarily caused by the Laplacian of small height or potential variations across a sample surface. This variation is blurred slightly to account for the interaction of the electrons with the electrical potential away from the surface. The method is derived from the established geometrical theory of MEM contrast, and whilst it loses quantitative accuracy outside its domain of validity, it retains a simplicity that enables rapid interpretation of MEM images. A strong parallel exists between this method and out of focus contrast in transmission electron microscopy (TEM), which allows a number of extensions to be made, such as including the effects of spherical and chromatic aberration.

We extend the theory of Laplacian image contrast in mirror electron microscopy (MEM) to the case where the sample is illuminated by a parallel, collimated beam. This popular imaging geometry corresponds to a modern low energy electron microscope equipped with a magnetic objective lens. We show that within the constraints of the relevant approximations; the results for parallel illumination differ only negligibly from diverging MEM specimen illumination conditions.

A wave optical treatment of surface step contrast in a low-energy electron microscopy (LEEM) is presented. The aberrations of an idealised LEEM imaging system are directly incorporated into a transfer function (TF) and image simulations of surface steps are evaluated in one and two dimensions. Under the special circumstances of a weak phase object, the simplified form of the contrast transfer function (CTF) is used to discuss LEEM image contrast and optimum defocus conditions.Keywords: Contrast transfer function; low-energy electron microscopy (LEEM); phase contrast imaging; surface steps. * Corresponding author. 855 Surf. Rev. Lett. 2009.16:855-867. Downloaded from www.worldscientific.com by MCGILL UNIVERSITY on 02/05/15. For personal use only.

Image simulation methods are applied to interpret mirror electron microscopy (MEM) images obtained from a movie of GaAs droplet epitaxy. Cylindrical symmetry of structures grown by droplet epitaxy is assumed in the simulations which reproduce the main features of the experimental MEM image contrast, demonstrating that droplet epitaxy can be studied in real-time. It is therefore confirmed that an inner ring forms at the droplet contact line and an outer ring (or skirt) occurs outside the droplet periphery. We believe that MEM combined with image simulations will be increasingly used to study the formation and growth of quantum structures.