Estimation of the strain state of GexSi1-x/Si strained-layer superlattices by double-crystal X-ray diffraction

20-period strained-layer superlattices of nominal composition and width Ge0.2Si0.8 (5 nm)/Si(25 nm) and Ge0.5Si0.5 (5 nm)/Si(25 nm) were studied by double-crystal X-ray diffraction. The Ge content x was determined by computer simulation of the diffraction features from the superlattice. This method is shown to be independent of the relaxation of the superlattice. Alternatively, x can be obtained from the measured difference Δa/a in lattice spacing perpendicular to the growth plane. It is sensitive to the relaxation. Comparing the results obtained in these two different ways, information about the relaxation of the superlattices can be obtained