Optical studies of luminescence mechanisms and processes in semiconductor functional nanostructures and novel materials

Abstract

In recent years, novel semiconducting materials and functional nanostructures have been extensively explored due to their promising wide applications ranging from solar cells in spacecraft to LED display in cellphone. However, our current understanding on some fundamental physical mechanisms such as radiative recombination and other internal optoelectronic processes in these new materials and nanostructures is far from enough to support their rapid development. In this thesis study, we aim at promoting our current understanding on luminescence mechanisms and internal processes of carriers in several novel luminescent nanostructures and materials by conducting comprehensive optical studies on GaInP based solar cells, coupled InGaN QW-QDs nanostructures, organic luminescent complexes, etc. Meanwhile, theoretical analysis and modelling are also done to gain a deeper insight into the phenomena and underlying mechanisms. The main results and findings of this study are summarized as below. The steady-state localized-state ensemble (LSE) luminescence model has been further generalized to analyze dispersive thermodynamics of localized carriers and their transient luminescence behaviors. Analytical expressions of transient luminescence trace, luminescence lifetime and nonradiative recombination lifetime for localized carriers have been derived in the newly developed model. And the model has been successfully applied to quantitative interpretation of temperature dependent time-resolved photoluminescence in various systems with localized states in literature. A direct optical observation of carriers’ distribution transition from local delocalized states to Anderson localized states in the GaInP base layer of a GaInP solar cell has been obtained from the measured variable-temperature and variable-injection-current electroluminescence signatures. The local delocalized states are ascribed to the extended states in local atomic ordering domains of alternative InP/GaP monolayers in the partially ordered GaInP base layer, while the Anderson localized states result from the disordering locations of Ga and In atoms in the remaining regions. Unusual significant enhancement of photoluminescence at high temperatures in coupled InGaN QW-QDs hybrid heterostructure has been investigated both experimentally and theoretically. Enhanced phonon-assisted tunneling probability with temperature and effective suppression of non-radiative recombination rate in the hybrid quantum structure are revealed to be responsible for the anomalous luminescence enhancement phenomenon. Triplet state harvesting in fluorescence of luminescent Cu(I) complexes has been investigated via examining temperature dependent steady-state and time-resolved photoluminescence spectra as well as doing model analysis to the experimental spectral data. Thermally activated luminescence mechanism transition from triplet emission to singlet emission is demonstrated.