Diffraction scattering at angles far from the Bragg angle and the structure of thin subsurface layers

It has been shown that by measuring the angular dependence of X-ray diffraction scattering far from the Bragg peak, information on the structure perfection of thin subsurface layers can be directly obtained. This is associated with the fact that the waves generated in the crystal bulk compensate one another, and the intensity of rocking-curve tails is due mainly to scattering in the subsurface layer. The typical thickness of a scattering layer is related to the deviation angle by a simple relationship: Δz≃ Lexω0/α, where α is the deviation angle of the specimen from the exact Bragg position, ω0 the diffraction maximum width, and Lex the extinction length. The method of three-crystal diffractometry permitted the observation for the first time with a conventional X-ray source of a distorted layer with a thickness of ~10 nm.