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 (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 L, with an increase in their density at high values of 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 (L less than or equal to 1.6ML), 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 L. For L greater than or equal to 1.6 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 L, the increase being faster for L greater than or equal to 1.6 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.