X-ray diffraction data have to be corrected by geometrical correction factors prior to any quantitative analysis. Here the case of grazing incidence X-ray diffraction measurements is considered, including the case of high exit angles. First, an approach taking into account the evolution of the diffracting area during an 3 scan is presented. From the calculation of the effective part of the sample surface that participates in the diffraction phenomena at each step of the scan, a more accurate correction factor than those commonly used is derived and the evolution of the line shape along a zero-width rod is explained. Secondly, the case of ®nite-width rods, under the point-like sample approximation, is considered: the in¯uence of the partial integration, as a result of the detector in-plane acceptance, of a rod with an anisotropic in-plane shape, is studied and leads to an analytical expression for the corresponding correction factor. Finally, a full numerical simulation is presented, which provides an alternative method for correcting the experimental intensities and shows in which conditions the previous formulae are no longer valid.

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A quantitative estimate of the In/Ga surface concentration ratio in ultrathin ͑In, Ga͒As strained layers, grown by molecular-beam epitaxy on a GaAs͑001͒ substrate, is obtained using grazing incidence x-ray diffraction and diffuse-scattering measurements. The commensurate 2ϫ3 reconstruction is interpreted as due to cation ordering in the surface unit cell, locking the surface composition at the value In 2/3 Ga 1/3 As. Incommensurate 2ϫn reconstructions with nϽ3 (nϾ3) are described in terms of indium-depleted ͑-enriched͒ surface layers characterized by a statistical distribution of faults in the ideal 2ϫ3 atomic arrangement. Within a defined temperature range 450-490°C, a unique correspondence between the incommensurability parameter n and the indium surface fraction is established on the basis of a formulation of the diffuse scattering distribution.