Self-aggregated InAs quantum dots in GaAs

We present a detailed study, both structural and optical, of GaAs/InAs/GaAs heterostructures for InAs nominal coverages $\mathrm{(L)}$ ranging from 0.6 to 3 ML. Planar transmission electron microscopy (TEM) provides direct evidence of the presence of InAs quantum dots (QDs) for all values of $\mathrm{L,}$ with an increase in their density at high values of $\mathrm{L.}$ Transverse TEM shows also that those QDs have mostly small base angles. Accordingly, the evolution of the optical properties of InAs/GaAs is investigated by photoluminescence (PL) and PL excitation measurements (PLE). A broad PL band is observed in all samples, which is ascribed to the recombination of heavy-hole excitons in the InAs quantum dots, observed with TEM. For thin coverages $\text{(L⩽1.6}\mathrm{ML}\mathrm{),}$ a narrow PL band is also observed, which is attributed to recombination of heavy-hole excitons in a two-dimensional (2D) InAs layer. The two bands shift to lower energy for increasing $\mathrm{L.}$ For $\text{L⩾1.6\hspace{0.5em}}\mathrm{ML},$ the QD band has a faster shift and exhibits a complex structure, while the exciton recombination in the 2D-InAs layer vanishes. Those features, as well as the PLE results, indicate that: (a) quantum dots are connected by a two-dimensional InAs layer, at least for thin InAs coverages, which allows an efficient carrier capture into the dots; (b) the dot size increases with $\mathrm{L,}$ the increase being faster for $\text{L⩾1.6\hspace{0.5em}}\mathrm{ML}$ because above this thickness the growth becomes completely three dimensional. A simple model explains the PL data and results in a dot geometry in agreement with the TEM measurements.