Origin of mosaic structure in relaxed Si_1-xGe_x layers

We report high resolution x-ray diffraction measurements of relaxed Si0.7Ge0.3 layers on (001) Si substrates. Strain was relieved either by a glide-limited mechanism in structures where the composition was changed abruptly or by a nucleation-limited mechanism in structures having a compositionally graded intermediate layer. We find that the broadening of the x-ray peak of the surface alloy layer is similar in both types of samples, although the threading dislocation densities ranged from 1011 cm-2 to 5×106 cm-2. The effect of the threading dislocations is masked by the mosaic structure caused by the network of misfit dislocations underneath the layer.