Formation mechanisms of agglomerations in high-density InAs/GaAs quantum dot multi-layer structures

The study explores phenomena that occur during the growth of multi-stacked quantum dots (MQD) InAs layers using thin GaAs spacers. An arrangement of plastically relaxed agglomerations that extend along the MQD structure with diameters of about 70-120 nm and separated every 200-400 nm are observed. These agglomerations hinder the regular development of vertically aligned QD columns, leaving only a regular density of QDs in the first layer. The generation of these agglomerations has been modelled based on two extreme cases: (i) conical-like, it presents a more contrasted base and its nucleation is related to the presence of two coalesced QDs. The progression of these agglomerations can be deactivated in the upper layers, probably to a decrease in surface stress as consequence of misfit dislocation formation in the lower layers; (ii) volcano-like, it presents a crater shape due to the collapse of the upper layers with a higher accumulation of In. It is proposed that the origin of this type of agglomerations is due to the formation of a quantum ring (QR) in the first layer. The initial concavity of the QR increases during successive GaAs/InAs deposition cycles as tendency of Ga to out-diffuse rises.