A theoretical study of carrier dynamics in semiconductor quantum dots
Recent progress in semiconductor growth and fabrication techniques has allowed semiconductor structures of increasing dimensional confinement to be realized. In quantum dot structures carriers are confined in all three dimensions and therefore exhibit a discrete set of energy levels. It is expected that optoelectronic devices incorporating these structures will have improved performance. This study focusses on self-assembled quantum dot structures. In this growth technique, the quantum dots are formed on top of a thin quantum well wetting layer which has been deposited on top of a bulk substrate. The whole structure is finally overgrown by bulk material. The wetting layer plays a crucial role in the operation of quantum dot optoelectronic devices. Initially carriers diffuse into the 2D wetting layer before either recombining or being captured by the self-assembled quantum dots. Once captured in the dots, they either relax into lower energy levels or are re-excited back to the wetting layer. Therefore the wetting layer supplies the quantum dots with a carrier reservoir and hence plays a large part in the carrier dynamics. In this thesis, we calculate the scattering rates for capture from the wetting layer into the quantum dots and the reverse process, the emission of a carrier from the dot into the wetting layer. We also calculate the carrier relaxation rates between quantum confined energy levels in the quantum dots. Using these scattering rates, we construct a rate equation model in order to describe the carrier dynamics of quantum dot structures. The model is used to predict the occupancy of the quanV vi tum dots as a function of time, and this allows us to calculate the time-varying optical gain and spontaneous emission rates. This type of model enables us to predict quantum dot laser dynamics including non-linear effects such as gain saturation and spectral hole burning. We also compare the results obtained from our model with time resolved photoluminescence experiments and see good qualitative agreement.