Strain relaxation in low-mismatched GaAs/GaAs _1-x Sb _x /GaAs heterostructures

The creation of structural defects in low-mismatched GaAs/GaAs _0.92 Sb _0.08 /GaAs(001) heterostructures and their evolution during strain relaxation have been studied using transmission electron microscopy as well as high-resolution x-ray diffraction and atomic force microscopy. These GaAsSb films had thicknesses in the range of 50–4000 nm with 50-nm-thick capping layers and were grown using molecular beam epitaxy. The strain relaxation had three distinct phases as the film thickness was increased, whereas the thin GaAs capping layers exhibited only the initial sluggish stage of relaxation in heterostructures with thick GaAsSb films. The character of the misfit dislocations at the two interfaces was determined using g.b analysis, and atomic-scale structural information was obtained using aberration-corrected electron microscopy. Stage-I relaxation took place primarily by glide of dissociated 60° dislocations. Although the films were mostly free of threading dislocations, many curved dislocations extended into the substrate side for heterostructures that had undergone Stage-II and Stage-III relaxation. Investigation of dislocation density evolution at the cap/film interface and morphological evolution of the growth surface revealed a strong correlation. The smoother growth surface in the heterostructure with 4000-nm-thick film resulted in a reduced areal density of surface troughs that acted as nucleation sites for dislocations, which explained the decreased dislocation density at the cap/film interface. Overall, these results prove that heterogeneously nucleated surface half-loops are the primary source of threading dislocations in low-mismatched heterostructures.