Dislocation contrast in X-ray reflection topography of strained heterostructures

Abstract

X-ray reflection topography for imaging misfit dislocations in highly strained systems with a small critical thickness is assessed. The samples often contain complex heterostructures such as capping layers, quantum wells or superlattices. For the first time the strain within the heterostructure is taken into account explicitly into simulation of X-ray reflection topographs. A new algorithm to simulate such heterostructures is proposed on the basis of a local kinematical approximation within one step of the numerical integration of the Takagi-Taupin equations. Dramatic effects of the heterostructure configuration on the contrast of dislocation images are predicted by simulation, based on three effects: firstly the depth of the dislocation (referred to the free surface) determines the effect of surface relaxation on the distortion field; secondly the composition around the dislocation determines which part of the dislocation field takes part in image formation; thirdly interference between different layers with identical composition modulates the rocking curve and hence the resulting contrast. This is confirmed experimentally for two In_xGa_1−xAs/GaAs structures, a capped layer with growing cap thickness and a single epitaxial layer on a substrate with a buried multiquantum well.