DEPTH-RESOLVED MEASUREMENT OF LATTICE-RELAXATION IN GA1-XINXAS GAAS STRAINED-LAYER SUPERLATTICES BY MEANS OF GRAZING-INCIDENCE X-RAY-DIFFRACTION/

The state of relaxation in two different superlattices (SLs) of a syst em with large lattice mismatch, Ga0.8In0.2As/GaAs grown on GaAs [001] by molecular beam epitaxy, has been investigated by surface-sensitive grazing-incidence diffraction (GID). The SL is squeezed between the su bstrate and a thick GaAs top layer. The thickness of individual GaInAs layers t(a) (active layer) is the same in both samples, while the GaA s barrier thickness t(b) is different. We have studied the influence o f the thickness ratio t(b)/t(a) on the state of relaxation for differe nt distances from the sample surface. We find that for thick barriers the whole SL remains coherently strained and for the thinner barrier t hickness the SL is partially relaxed against the the GaAs top layer. T he GID technique was applied for the first time to obtain depth resolu tion of the lateral lattice parameter in a SL. It is demonstrated to b e especially well suited for SL systems with a small difference of the average electron density between the sublayers. The scattering contra st is improved by measuring the intensity as a function of the exit an gle (''rod scans'') from the ''weak'' (200) Bragg reflection. Comparin g computer simulations with the measured variation of the scattering c ontrast between GaAs and GaInAs layers obtained from different ''infor mation depths'' and at different angular positions of the in-plane roc king curves, the state of relaxation can quantitatively be evaluated. On the basis of these results we propose two models for the partial re laxation of the SL into the state of strain-reduced domains. We believ e that the partial relaxation is due to the elastic field interaction between the GaInAs layers accross the GaAs barriers, if t(b) is small.