Ring-to-dots transformation of InGaAs quantum rings grown by droplet epitaxy

Post-growth annealing in UHV can lead to the deformation of crystallized nanostructures. Annealing effects on structural and optical properties of InGaAs quantum rings (QRs) grown by droplet epitaxy was examined. The InGaAs QRs were fabricated by depositing 3 monolayer (ML) of In0.5Ga0.5 on GaAs (100) at the substrate temperature of 140 degrees C, and crystallized in As-4 flux of similar to 7 x 10(-6) Torr at 180 degrees C for 5 min. After the crystallization, the substrate temperature was ramped up to the desired annealing temperature (T-a) of 300-400 degrees C in As-4 beam. In situ transformations of surface morphology were observed upon the evolution of RHEED patterns. Surface morphology was analyzed by AFM. As ramping up the annealing temperature, the QRs deformed and changed to numerous smaller QDs about the QR positions. Supposedly, there was segregation of group III atoms out of the QRs. At 380 degrees C the QRs properly lost their actual shapes and burst into high-density small QDs. Most possible reasons of the segregation can be crystalline instability of the low-temperature-crystallized QRs, along with the different surface kinetics of In and Ga atoms. (C) 2013 Elsevier B.V. All rights reserved.