Spectroscopy of size dependent many-particle effects in single self-assembled semiconductor quantum dots

Single InAs quantum dots (QDs) grown with the Stranski-Krastanov method in a InGaAs quantum well embedded in GaAs and emitting in the near infrared have been optically investigated. Photoluminescence of the QDs was excited non resonantly with a He-Ne laser and single dot spectroscopy was carried out at temperatures below 60 K. To perform QD spectroscopy at low temperatures a very stable micro-photoluminescence microscope set-up fully integrated in a liquid helium cryostate has been developed. The experimental set-up allows mapping of the optical emission by recording spectra for every point of a scan grid. This mapping mode is used to acquire optical images and to locate a particular dot for investigation. At low excitation power a single sharp line arising from recombination of a single exciton in the dot is observed. Varying the excitation density the spectra become more complex, with appearance of the biexciton emission line, followed by emission from excitons occupying higher shells in the dot. A statistics of biexciton binding energies over a dozen of dots was acquired and the results compared with single QD spectroscopy data available in the literature, for III-V material systems. Despite the impossibility to obtain precise information about the dimensions of the particular QD under investigation, from the analysis of these data a general trend can be found in the dependency of the biexciton binding energy on the size of InAs QDs, parameterised with their emission energy.