Influence of bimodal distribution and excited state emission on photoluminescence spectra of InAs self-assembled quantum dots

Optical properties of self-assembled quantum dots (SAQDs) samples were investigated through photoluminescence (PL) with variation of temperature and excitation density. Two types of InAs SAQDs were studied: dots grown directly on GaAs, and dots grown inside an InGaAs quantum well, heterostructure known as dot-in-a-well (DWELL). The QDs structures were produced through conventional growth (with several deposition rates) as well as through pulsed growth mode, which consists in deposition cycles with growth interruption intervals. In the samples of QDs grown on GaAs, two PL peaks were identified and their behavior as a function of temperature can be explained by taking into account a bimodal distribution of dot sizes. For the DWELL sample only one PL peak was identified, which was assigned to the emission of the ground state of QDs belonging to a larger Gaussian distribution of dot sizes. SAQDs samples showed an anomalous behavior of the peak energy, integrated intensity and linewidth (FWHM) of the PL signal, as a function of temperature. To explain the observed results, a model was proposed that takes into account the size distribution of dots and the excited state emission of the family of larger dots. The QDs can be distributed in a bimodal size distribution, and the emission from the excited state can be observed, or not, depending on the growth conditions and on the potential profile of the dot. Such aspects can be combined to explain several peculiarities of the PL spectra observed in this and other published papers on SAQDs optical properties.