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

Abstract

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.