Quantification of lattice images: the contribution from diffuse scattering

At present, quantitative comparisons of experimental and simulated high-resolution images show that the contrast in experimental images is usually much less than is predicted by simulations, typically by a factor of around three. Investigations of this contrast discrepancy in images of amorphous carbon have shown that the contrast in experimental images is lower than in simulations by the same factor for a wide range of spatial frequencies, suggesting the possibility that the contrast loss is due to the addition of a constant background to the experimental images. The source of this constant background is investigated using convergent-beam diffraction patterns as a function of thickness and lattice images from an [001]-oriented crystal of GaAs. The diffuse background is measured in the convergent-beam patterns and found to contribute about 33% of the total intensity at 25 nm thickness. However, at this thickness the experimental lattice image contrast is only about half that of a Simulated lattice image. Thus, although the measured diffuse scattering produces a significant contrast reduction in lattice images, the diffuse scattering from phonons and amorphous carbon is not sufficient to explain why lattice images have such low contrast.