COMPOSITION DETERMINATION FROM HREM IMAGES OF SUBSTITUTIONAL ALLOYS

The relation between the contrast in an HREM image of a substitutional alloy and the local composition is studied within the framework of the real-space theory of electron scattering. Two algorithms are derived to determine the composition: a deconvolution and a convolution. The algorithms are tested on simulated images of AlxGa1-xAs, of a GaAs/AlAs interface and of an AlxGa1-xAs alloy with randomly occupied group III sites. The deconvolution method is very sensitive to small deviations from the linear relationship between image contrast and composition, and to the influence of neighbouring columns. The more robust convolution method works best for defocus values optimizing the differences in contrast between GaAs and AlAs images, and for sample thickness around 10 nm. It is estimated that under optimal imaging conditions compositions can be determined within separate columns of group III atoms with an accuracy of ±0.10. © 1990.