Depth-resolved measurement of lattice relaxation in Ga1−xInxAs/GaAs strained layer superlattices by means of grazing-incidence x-ray diffraction

Abstract

The state of relaxation in two different superlattices (SLs) of a system 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 substrate and a thick GaAs top layer. The thickness of individual GaInAs layers ta (active layer) is the same in both samples, while the GaAs barrier thickness tb is different. We have studied the influence of the thickness ratio tb/ta on the state of relaxation for different distances from the sample surface. We find that for thick barriers the whole SL remains coherently strained and for the thinner barrier thickness the SL is partially relaxed against the the GaAs top layer. The GID technique was applied for the first time to obtain depth resolution of the lateral lattice parameter in a SL. It is demonstrated to be especially well suited for SL systems with a small difference of the average electron density between the sublayers. The scattering contrast is improved by measuring the intensity as a function of the exit angle (‘‘rod scans’’) from the ‘‘weak’’ (200) Bragg reflection. Comparing computer simulations with the measured variation of the scattering contrast between GaAs and GaInAs layers obtained from different ‘‘information depths’’ and at different angular positions of the in-plane rocking curves, the state of relaxation can quantitatively be evaluated. On the basis of these results we propose two models for the partial relaxation of the SL into the state of strain-reduced domains. We believe that the partial relaxation is due to the elastic field interaction between the GaInAs layers accross the GaAs barriers, if tb is small.