Investigations of Electronic Structure and Optical Properties of Ii-Vi Self-assembled Quantum Dots

Download or read book Investigations of Electronic Structure and Optical Properties of Ii-Vi Self-assembled Quantum Dots written by Tuan Anh Nguyen and published by . This book was released on 2006 with total page 218 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this dissertation, we use different optical and imaging spectroscopy techniques to study electronic structure and optical properties of CdTe/ZnTe and CdSe/ZnSe self-assembled quantum dots (SAQDs). We perform single dot photoluminescence excitation experiments to identify carrier excitation mechanisms in CdTe/ZnTe QDs. The first mechanism is direct excitation into the QD excited states followed by relaxation to the ground state and the second mechanism is direct excitation into the QD ground states through LO phonon-assisted absorption. We then execute resonant PL measurements for both CdTe and CdSe QD ensembles to study the dependence of exciton-LO phonon coupling on QD size in these II-VI SAQDs. We shown that the strength of exciton-LO phonon coupling increases significantly for QDs with lateral sizes smaller than the exciton Bohr radius (e.g. as-grown CdTe QDs) while for larger QDs (e.g. CdSe or CdTe annealed) it is almost independent of the QD emission energy, and therefore presumably of the QD size. In order to study electronic coupling between SAQDs, we setup imaging experiments with the use of a hemisphere solid immersion lens. While the PLE imaging measurements show the existence two-dimensional platelets with a typical size of about 500 nm which provide spatially extended but strong localized states through which different QDs could be populated simultaneously, the spatially resolved imaging data demonstrates a complete 2D map of those platelets. These results are further supported by computational calculations based on finite element analysis. Low temperature exciton spin relaxation in symmetric CdTe SAQDs has been thoroughly studied by means of cw polarized magneto-PL and polarized time-resolved PL spectroscopies. We find that the degeneracy of exciton energy levels has a strong influence on the spin transition. When the exciton spin states in QDs are degenerate, the spin relaxation time is much shorter than the exciton recombination time. In contrast, if this degeneracy is removed, either by asymmetry or an external magnetic field, the spin relaxation time becomes much longer than the exciton recombination time. Using simple rate equation models, we estimate exciton spin relaxation times equal to 4.8 ns and 50 ps for non-degenerate and degenerate QD states, respectively.