Detecting Spatially Localized Exciton in Self-Organized InAs/InGaAs Quantum Dot Superlattices: a Way to Improve the Photovoltaic Efficiency.

This paper reports on experimental and theoretical investigations of atypical temperature-dependent photoluminescence properties of multi-stacked InAs quantum dots in close proximity to InGaAs strain-relief underlying quantum well. The InAs/InGaAs/GaAs QD heterostructure was grown by solid-source molecular beam epitaxy (SS-MBE) and investigated via photoluminescence (PL), spectroscopic ellipsometry (SE), and picosecond time-resolved photoluminescence. Distinctive double-emission peaks are observed in the PL spectra of the sample. From the excitation power-dependent and temperature-dependent PL measurements, these emission peaks are associated with the ground-state transition from InAs QDs with two different size populations. Luminescence measurements were carried out as function of temperature in the range of 10-300 K by the PL technique. The low temperature PL has shown an abnormal emission which appeared at the low energy side and is attributed to the recombination through the deep levels. The PL peak energy presents an anomalous behavior as a result of the competition process between localized and delocalized carriers. We propose the localized-state ensemble model to explain the usual photoluminescence behaviors. The quantitative study shows that the quantum well continuum states act as a transit channel for the redistribution of thermally activated carriers. We have determined the localization depth and its effect on the application of the investigated heterostructure for photovoltaic cells. The model gives an overview to a possible amelioration of the InAs/InGaAs/GaAs QDs SCs properties based on the theoretical calculations.