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Grazing‐incidence x‐ray diffraction (GIXD) permits the direct measurement of in‐plane lattice parameters of SiGe films that are too thin to yield good results from normal‐geometry triple‐axis techniques. A unique ‘‘X’’‐shaped pattern has been seen in H–K reciprocal space maps of diffracted x‐ray intensity from SiGe films that have relaxed via a modified Frank–Read mechanism. Contours of intensity are seen along the 〈110〉 directions from the (4¯00) reciprocal lattice peak with the introduction of the first dislocations. For higher dislocation densities the X‐shaped contours are anisotropically distorted and a satellite peak, corresponding to the lattice parameter for a partially relaxed film, becomes identifiable at lower H. In contrast, H–K reciprocal‐space contours from thin SiGe films that have relaxed by roughening and subsequent random nucleation of dislocations display broad, oval‐shaped contours centered at the (4¯00) reciprocal lattice point for the film. Numeric simulations of GIXD from a variety of dislocation arrangements were performed in order to understand the origin of the X pattern. We show that this pattern arises from an array of long misfit dislocations running in the 〈110〉 directions. The anisotropic distortion of the X pattern arises at higher dislocation densities from orthogonal intersections of dislocations with equal Burgers vector, which are characteristic of dislocation networks generated by the modified Frank–Read mechanism. We also verify that the measured values of the in‐plane lattice parameter, together with the out‐of‐plane lattice parameter determined from the symmetric (004) reflection, lead to accurate calculation of the composition and strain in these SiGe layers. © 1996 American Institute of Physics.