Recombination dynamics of excitons and exciton complexes in single quantum dots

We have studied the recombination dynamics of excitons and excitonic complexes confined in single GaAs quantum dots, embedded in a type-II GaAs/AlAs bilayer, formed at unintended growth imperfections. The small density of defects leads to the spatial isolation of the quantum dots, allowing to address individual specimens without any further sample processing. Any influence of carrier diffusion on the recombination dynamics is avoided by using quasi-resonant excitation, below the quantum dot barrier. Under these excitation conditions, the recombination occurs within a 2 nanosecond time window since relaxation takes place only inside the quantum dot. At low powers, the photoluminescence spectra are dominated by very sharp lines attributed to the exciton and the bi-exciton/charged-exciton transitions, while at large powers it is possible to observe the emission from higher-order exciton complexes. We have found a retardation of the emission increasing the pump power and interpreted it as an evidence for a sequential decay of the different excitonic species