Principles and Techniques of Transmission Imaging of Surfaces

Abstract

Although many surface characterisation techniques are essentially broad beam methods yielding diffraction or other information averaged over large areas of surface, crucial properties of the surface may depend on steps or other inhomogeneities. There is indeed an instability, shown schematically in figure 1, whereby even a uniform single crystal overgrowth such as an expitaxial layer or adsorbed oxide tends to become inhomogeneous, forming island structures or localised defects. The overgrowth has an energy E proportional to thickness t at small t where it matches its lattice parameter with the substrate crystal at the cost of elastic strain energy (point A). At larger values of t, when elastic strain energy becomes prohibitive, misfit dislocations are introduced to relieve it. Eventually where the overgrowth has completely relaxed to its own lattice parameter, E becomes the characteristic interfacial energy of the boundary, independent of t (point C). Since the E versus t curve is concave downwards, a homogeneous film represented by point B can reduce its energy to the point D by breaking up into regions A and C.