A study was made of X-ray diffraction topographs, of extremely pure and perfect single crystals, which had been made by using low-energy undulator radiation from a positron storage ring. The typical defect images which were observed were rather large round ones with black-white contrast and a diameter of about 40μm. By using the dynamical theory of X-ray diffraction, defect contrast was explained in terms of tensile strain in the lattice around voids close to the exit surface. This discovery of void-like micro-defects partly explained the reduced density of crystals which were intended for metrological purposes.

Dynamic Diffraction Imaging of Voids in Nearly Perfect Silicon. T.Tuomi, R.Rantamäki, P.J.McNally, D.Lowney, A.N.Danilewsky, P.Becker: Journal of Physics D, 2001, 34[10A], 133-5