Observação direta e estudo da difração bragg paralela a superficie de monocristais

AUTOR(ES)
DATA DE PUBLICAÇÃO

1991

RESUMO

In this work a new scattering geometry is developed to allow for direct observation of the X-ray beam, diffracted parallel to the crystal surface. It is a modification of the usual Renninger geometry, for systematic multiple diffraction occurrence. While in Renninger geometry the primary incidence plane lies on the diffractometer plane in the proposed geometry, the secondary incidence one is made parallel to the diffractometer plane, instead. The geometric study in reciprocal space for occurrence of surface parallel diffraction, provides the Miller indices selection, for [100], [110] and [111] surface orientation, in cubic crystals. For intensity distribution purposes, it was selected two dislocation free Silicon crystals, Si [111] and Si [100], with about 1 cm of surface diameter. Topographic section experiments were obtained, for the parallel surface diffracted beam using values of mt » 7, with MoKa radiation. Three surface parallel diffraction cases were examined in this study, Si [111] simple diffraction 000 113, Si [111] three beam multiple diffraction 000 222 113 and Si [100] four beam multiple diffraction 000 400 220 220. An intensity enhancement for the beam, coming from the sample surface region, was found in all cases. On these topographies one can clearly observe the intensity distribution of the underneath portion of the beam. A brief discussion is made on the new multiple Borrmann effect, parallel to the crystal surface. This effect can be compared with the one occurring perpendicularly to the surface for the same crystal and same multiple diffraction case [0-40]. In order to explain the intensity enhancement at the crystal surface, calculations of absorption coefficient, wave field and intensity profile, using Laue formulation of Ewald dynamical theory [00-1] were made. This development was applied to simple diffraction case, in Laue symmetric and surface parallel diffraction, using Silicon and MoKa radiation. The results are able to account for the existence of the surface enhancement, and bears the same explanation as the one presented by thin crystals Laue symmetric diffraction [0-27], at the extremes of Borrmann fan. Strong beam intensity was found for 462 reflection in InP [001] samples, using MoKa radiation. The same was obtained for 062 reflection in InP [001] samples using CuKa radiation, for value6 of mt >200. The 4-fold equivalent crystal settings, in case of the three beam multiple diffraction 000 004 062, provided the expected equal intensities. The existence of intensities for such very high crystal thicknesses has not been reported in the literature yet. Topographic sections were obtained with same conditions for several InP [001] samples and for InP layered material. Striking results were found for crystal under stress and also thin layers

ASSUNTO(S)

raios x - difração

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