Nucleation and diffusion processes during the stacking of bilayer quantum dot InAs/GaAs heterostructures

The growth front stacking of bilayer quantum dot (BQD) InAs/GaAs heterostructures was studied by reflection high-energy electron diffraction (RHEED). The mean-field theory was employed to describe the quantum dots (QDs) nucleation, which was experimentally monitored during the intensity changes of the (002)-RHEED diffraction spot along the two- to three-dimensional (2D-3D) InAs growth mode transition. The diffusion parameter obtained from fits of the 2D-3D transition curves was associated to the rate of atoms supply from 2D and 3D islands precursors. The variation of the nucleation parameters during the vertical alignment of QDs associated to the coupling of strain fields were related to the changes of the QDs size and the wetting layer thickness. Numerical simulations indicated that these changes reduce the strain in the BQD heterostructures. Damped oscillatory behavior was observed for the InAs/GaAs critical thickness (H-c) as a function of the number of BQD. The bilayer number after which H-c did not vary significantly, coincided with the one without important variation of the diffusion parameters. The number of BQD layers required to reach this quasi-equilibrium condition depends on the growth parameters of the first layer and the spacer layer thickness, as supported by numerical simulations.