A systematic analysis of HREM imaging of sphalerite semiconductors

The interpretation of high-resolution electron micrographs of semiconductor compounds having the sphalerite structure has been investigated both theoretically using extensive computer simulations and experimentally. The images display considerably more complexity and diversity than those of the diamond-structured elemental semiconductors, mainly due to the absence of a crystallographic center of symmetry. A systematic study of image contrast for these compounds reveals a dependence on the degree of their non-centrosymmetry. Subtle contrast effects result from the differences in both amplitude and phase of Bijvoet pairs of reflections that appear with increasing thickness, due to the breakdown of Friedel's law under dynamical diffraction conditions. A strategy is suggested for identifying the absolute direction of the polar axis in low-resolution images. It is shown that first- and second-order scattering effects can be used to determine the crystal polarity from the fine detail visible in experimental micrographs of sufficiently high resolution.