Raman spectroscopy of in situ annealed InAs/GaAs quantum dots

Nonresonant Raman scattering measurements were performed on a three-layered sample of in situ annealed InAs/GaAs self-assembled quantum dots. The thermal annealing step was done during the growth of each GaAs spacer layer, where the substrate temperature was raised from 530 deg. C to 580 deg. C as measured by a pyrometer. Three Raman signals are clearly seen at 264 cm-1, 268 cm-1, and 271.5 cm-1 which can be attributed to the longitudinal optic (LO) phonons of the dots. These three different signals are attributed to different degrees of In/Ga alloying or intermixing in each dot layer as a consequence of the different thermal treatments that each layer was subjected to. The Raman signal of the wetting layer (WL) is remarkably weak and suggests erosion of the wetting layer. In contrast, Raman scattering measurements on an unannealed sample show two overlapping features at 259.5 cm-1 and 261.5 cm-1, which are attributed to the LO phonons of the dots. The WL signal is clearly observed at 251.5 cm-1. To support our Raman observations, the appearance of a strong higher-energy peak in photoluminescence measurements suggests that alloying did occur in the annealed sample