The last few years have een rapid improvements in semiconductor growth techniques which have produced an expanding range of high quality heterostructures for new semiconductor devises. As the dimensions of such structures approach the nanometer level, it becomes increasingly important to characterise materials properties such as composition uniformity, strain, interface sharpness and roughness and the nature of defects, as well as their influence on electrical and optical properties. Much of this information is being obtained by electron microscopy and this is also an area of rapid progress....