Ray Topography and Interferometry

X-ray topography lacks the simplicity of etch patterns and the high resolution of transmission electron microscopy. However, the latter investigations are confined to thin films, and etch pits are not always associated with dislocations. Topographic techniques provide an excellent method for characterizing dislocation content of high-quality crystals up to a few millimeters thick. 

Crystals oriented to satisfy Bragg s law are used as optical components in X-ray interferometry. The interference patterns are sufficiently sensitive to small displacements to allow the observation of strains and defects in crystals. Lattice rotations of 10 rad or dilations dd/d of 10 can be detected. Inhomogeneous strain produces moire fringe patterns showing dislocations and the strain-associated impurities in concentrations below one in 10 . 

Although the physics of these devices is simple, construction and operation of the X-ray interferometer had to await the arrival of large, perfect single crystals such as semiconductor silicon. The interferometer makes use of the fact that the refractive index for X rays is slightly less than unity, about 1 — 10 . This makes conventional optical techniques difficult because the focal length of a lens would be several kilometers, but for interferometry, where optical paths are balanced, the physical dimensions need only be accurate to X — n) 50 /rm. 

Bragg reffection occurs over a very narrow angle ( 10 rad) for perfect crystals. The degree of perfection determines the relative phases of beams generated by the X-ray interferometer. This sensitivity permits measurement of crystal strains as well as shifts caused by objects placed in the path. 

In addition to characterization work, interferometers can be used to measure X-ray scattering factors, to do phase contrast microscopy, and for absolute measurements of lattice spacings. 

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