Coherence length of strain relaxation mechanisms in InGaAs/GaAs [001] graded buffer layers

A new phenomenon of strain relaxation will be presented. A curvature of the epi-layer lattice has been detected in a series of InGaAs buffer layers grown on non intentionally off cut (001) GaAs substrates with different grading composition layers. It consists of a lateral dependence of the layer lattice orientation with respect to the substrate which varies coherently long the sample surface on the scale of several mm. The coherence length has been found to depend on the compositional profile and on the sample dimension. When it is smaller than substrate dimension, the curvature develops within domains bounded by tilt discontinuities. A reduction of the mechanism leading to curvature can be obtained optimizing the growth conditions. The origin of the epilayer curvature has been addressed to a lateral distribution of the Burgers' vectors.