X-Ray anomalous transmission and topography of impurity clustering in perfect crystals

Abstract

In anomalous transmission two plane waves of X-ray intensity are coupled in the crystal. Their interaction produces nodes in intensity coincident with atomic planes. When this occurs there is a very large reduction in the intensity of photoelectric absorption and à corresponding increase in the amount of transmitted X-rays. Any deformation of the lattice which moves atoms away from the atomic planes will reduce the intensity transmitted. Experiments on initially highly perfect heat treated silicon crystals show that the intensity of anomalous transmission can be reduced to zero even in crystals presumably free of oxygen. The kinetics of the heat treatment effect depend on the initial concentration of oxygen in the crystal. More recently carefully grown crystals in which care had been taken to exclude oxygen show no decrease in anomalous transmission after heating at 1 000° C. These experiments allow us to conclude that the effects observed earlier are due to oxygen and not to vacancies or other impurities. In all cases the initial perfection of the crystal can be restored by heating at 1 350° C to dissolve the clustered oxygen. The effects of the heat treatment have been followed by X-ray topography. The importance of section topographs in studies of this nature will be discussed. The treatment at 1 000° C produced dark unresolved contrast on X-ray section topographs and leads to a rapid obliteration of the normal Pendellösung fringe patterns observed in perfect crystals. Higher temperature heat treatment at 1 200° C produces two main types of defects that can be easily resolved by X-ray topography : prismatic dislocation loops and stacking faults. The topographic evidence for these defects will be briefly described. The X-ray topographic work is consistent with earlier results by transmission electron microscopy which reveal the same kind of defects on a much smaller scale. The anomalous transmission and the topographic results are compared with recent theory which ascribes the reduced anomalous transmission intensity to increased photoelectric and diffuse scattering absorption.