Stress evolution during ripening of self-assembled InAs/GaAs quantum dots

We have investigated the annealing behavior of InAs quantum dots (QDs) grown on GaAs(001) substrates by molecular beam epitaxy. An in situ cantilever beam setup was employed to record the stress evolution during deposition and subsequent annealing at the growth temperature. Ex situ atomic force microscopy (AFM) was used to characterize the morphological evolution of the QDs. During growth of InAs QDs, a stress of 4.05 +/- 0.5 GPa develops in the wetting layer. Beyond a critical thickness of 1.5-1.6 monolayer, the strain is relieved by the QD formation. During subsequent annealing the build-up stress relaxes. For annealing at temperatures around 440 and 470 degrees C, QDs undergo standard ripening. Models based on different mechanisms for Ostwald ripening, namely, kinetic and diffusion limited, are developed and fitted to the stress relaxation' curves. Although the relaxation curve for annealing at 440 degrees C can be fitted reasonably well with all models, the model describing ripening limited by diffusion along dot boundaries yields a slightly better fit. On the other hand, at 455 and 470 degrees C, the relaxation curve can be fitted very well only with the model in which ripening is controlled by attachment/detachment of atoms on the dot surface. For samples grown and annealed at 500 degrees C, the stress accumulated during QD formation relaxes below the value which was built up by wetting-layer growth. AFM images taken at different annealing stages reveal that the QDs ripen first and then dissolve after 7.5 - 10 min annealing. These observations are explained by a combination of In desorption and interdiffusion between Ga and In. (c) 2006 American Vacuum Society.