Self-assembled quantum dots with tunable thickness of the wetting layer: Role of vertical confinement on interlevel spacing

Epitaxial self-assembled quantum dots (QDs) are commonly obtained by the Stranski-Krastanow (SK) growth mode, in which QDs form on top of a thin two-dimensional (2D) wetting layer (WL). In SK QDs, the properties of the WL such as thickness and composition are hard to control independently of those of the overlying QDs. We investigate here strain-free GaAs/AlGaAs QDs located under a GaAs quantum well (QW), analogous to the WL in SK QDs. The thickness of such a QW can be arbitrarily controlled, allowing the optical properties of the QDs to be tuned without modifying the QD morphology and/or composition. By means of single-QD photoluminescence spectroscopy, we observe well-resolved excited-state shell structures with intershell spacing increasing monotonically with decreasing QW thickness. This behavior is well reproduced by eight-band k.p calculations combined with the configuration-interaction model taking the realistic QD morphology as input. Furthermore, for the thinnest GaAs layer investigated here, no QW emission is detected, indicating that it is possible to suppress the two-dimensional layer usually connecting QDs. Finally, we find that all recombination involving an electron-hole pair in the ground state, including the positive trion, occurs at the low-energy side of the neutral exciton emission. This behavior, previously observed for GaAs/AlGaAs QWs, is a consequence of the large lateral extent of the QDs, and hence of pronounced self-consistency and correlation effects.