Morphology and relaxation in InyGa1-yAs/GaAs multi-layer structures

Metastable InGaAs/GaAs multi-layer structures (MLS) on GaAs substrates with a constant interfacial strain and number of periods varying from 14 to 75 are studied by high-resolution X-ray reciprocal-space maps (HRXRSM), transm ission electron microscopy, and differential contrast microscopy. These cha racterization techniques are used to monitor both the magnitude of relaxati on and the change in crystalline state from a coherent "pseudomorphic" grow th to a dislocated condition as the number of periods increases. For 14-per iods, the diffuse intensity emanating from the entire MLS region arises fro m periodic, coherent crystallites. For 17- and 30-period, the displacement fields around the active region transition to coherent random crystallites (mosaic blocks). At 50-periods, displacement fields of overlapping dislocat ions from relaxation of the random crystallites cause the initial stages of relaxation of the active region. At 75-periods, relaxation of the strained region becomes bi-modal characterized by overlapping dislocations caused b y mosaic block relaxation and periodically spaced misfit dislocations forme d by 60 degrees -type dislocations. The relaxation of the MLS has an expone ntial dependence on the diffuse scatter length-scale which is shown to be a sensitive measure of the onset of relaxation. (C) 2001 Published by Elsevi er Science B.V.