Optical properties of bilayer InAs/GaAs quantum dot structures: Influence of strain and surface morphology

Double layers of InAs quantum dots (QDs) separated by 112-Å GaAs spacer layers have been grown by molecular-beam epitaxy on GaAs(001) substrates. Photoluminescence measurements reveal that the emission of the second QD layer is blueshifted with respect to the first one. The two peaks can be made coincident by increasing the amount of InAs deposited in the second layer and also by annealing the spacer layer. Scanning tunnelling microscope measurements of the starting surface and of the uncapped QD’s have been used to interpret these results. The QD’s in the two layers are shown to have the same composition (pure InAs) before capping. However, QD’s with similar shapes and volumes exhibit a blueshift in emission when grown in the second layer compared to a single layer. This is attributed to enhanced intermixing during the capping stage of the second-layer QD’s, and is a consequence of these dots being more strain-relaxed due to the strain fields associated with the first QD layer. The blueshift is smaller for annealed spacer layers due to a lesser degree of strain relaxation as a result of the change of surface morphology induced by the annealing process.