Kinetics of the evolution of InAs/GaAs quantum dots to quantum rings: A combined x-ray, atomic force microscopy, and photoluminescence study

We present an experimental study of the evolution of InAs/GaAs quantum dots partially capped with GaAs, as an annealing process transforms them first into quantum rings and later into holes penetrating the whole cap layer. Shape, composition, and optical emission were monitored as a function of annealing time by means of atomic force microscopy, x-ray photoemission microscopy, and photoluminescence, respectively. Our results show a progressive dissolution of the original dot, with the outdiffused material forming a second wetting layer on the planar region surrounding the dot. For the longest annealing time, in a situation close to thermodynamic equilibrium, no residual dot material is left in the holes, and an In-rich layer covers uniformly the surface. Our findings were examined by taking into account the surface chemical potential previously written for Stranski-Krastanov islands partially covered with a cap layer. We show that the energy gain due to the formation of the second wetting layer is the driving force for the shape and composition evolution, but its analytical expression should be modified, with respect to previous formulations, by taking into account the observed material interdiffusion.