Nonstoichiometric low-temperature grown (LTG) GaAs0.6Sb0.4 is epitaxially grown by molecular beam epitaxy on a lattice mismatched Al0.77In0.23As buffer layer. Ex situ annealing leads to an increase in the wafer sheet resistivity. Values as high as 1.7 x 10(8) Omega/sq were measured. By high resolution transmission electron microscopy, clusters were observed in LTG-GaAs0.6Sb0.4 after annealing, some of them attached to dislocation lines. Moreover, in a 600 degrees C annealed sample, the clusters have two different crystal structures and a spread in cluster size is present from an early formation state tip to a diameter of 12.5 nm. Hence, the strain surrounding the clusters is not uniform, which leads to an asymmetry of the x-ray diffraction (XRD) peak. In addition to an increased peak asymmetry with increasing annealing temperature, the XRD peak is shifted towards higher 20 values, resulting in a lattice constant shift of 0.24%. This lattice constant shift is due to a strain relaxation process by forming clusters. (c) 2006 American Vacuum Society.