ATOMIC RESOLUTION Z-CONTRAST IMAGING OF INTERFACES

Z-contrast imaging of crystals at a major zone axis is accurately described as a convolution of the scanning probe intensity profile with an object function sharply peaked at each atomic column. The object function contains the high-angle scattering cross sections and all dynamical diffraction effects, which manifest simply as a channeling effect for each individual atomic column. The channeling effect can be described entirely using highly localized s-type Bloch states. A tight binding formulation shows clearly that neighboring columns can be considered independently, in which case no proximity effects occur at interfaces, and it is possible to assemble object functions for interfaces, superlattices or complex unit cells from appropriate isolated string strengths which may be calculated using simpler model unit cells. Channeling effects are always less sensitive to column composition than the high-angle cross sections. Therefore, dynamical corrections to an image are always second order, and a first order intuitive interpretation can be obtained directly from the image, with no a priori assumptions on likely interfacial arrangements. These ideas will be illustrated for superlattices of SimGen and (YBa2Cu3O7-x)m (PrBa2Cu3O7-x)n.