Structural and radiative evolution in quantum dots near the InxGa1-xAs/GaAs Stranski-Krastanow transformation

The evolution of Stranski-Krastanow (SK) quantum-dot (QD) formation in ternary In0.6Ga0.4As/GaAs was studied with graded structures grown via organometallic vapor-phase epitaxy. Surface-probe microscopy showed island evolution between 3.5- and 6.5-monolayer (ML) deposition. Island densities increased exponentially (over three decades with 0.2-ML deposition) before saturation similar to 4.7 ML. Photoluminescence (PL) of capped structures show that the wetting-layer PL energy does not shift beyond the onset of the SK transition. PL intensities increased with QD concentration but not in proportion to QD density. After saturation, a sharp drop in PL intensity was observed, which we attribute to island coalescence and incoherent island formation. Excitation power dependence of the luminescence at different stages of QD evolution indicates a concentration dependence of optical saturation in self-forming InxGa1-xAs QD's.