Morphology and relaxation in In yGa 1− yAs/GaAs multi-layer structures

Abstract

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), transmission electron microscopy, and differential contrast microscopy. These characterization techniques are used to monitor both the magnitude of relaxation and the change in crystalline state from a coherent “pseudomorphic” growth to a dislocated condition as the number of periods increases. For 14-periods, the diffuse intensity emanating from the entire MLS region arises from 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 dislocations 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 by mosaic block relaxation and periodically spaced misfit dislocations formed by 60°-type dislocations. The relaxation of the MLS has an exponential dependence on the diffuse scatter length-scale which is shown to be a sensitive measure of the onset of relaxation.