Recombination and spin dynamics of excitons in indirect (In,Al)As/AlAs quantum dots

Abstract

In recent years semiconductor QDs have succeeded to perform the step from sample structures in fundamental research, with the aim to implement the spin state as a controllable size, to real device related applications, especially in the role of efficient light emitters. As the original aim has been proven to be difficult to fulfill and control reliably, new concepts for the further development of these structures had to be introduced. In the scope of the last years, the idea of a combining concept has been developed. QDs on the one hand have naturally an increased exciton recombination time, and further to that significantly longer spin relaxation times. On the other hand, classical indirect semiconductors show additionally increased exciton lifetimes, while their spin-orbit interaction is greatly reduced, making spin states more stable and longer-living. The combining concept therefore aims toward an indirect semiconductor with the additional limitations of a zero-dimensional structure. Based on this concept, the indirect (In,Al)As/AlAs QDs have been constructed, possessing a type I band alignment but having an indirect band gap in the momentum space.Main issue of the dissertation at hand is to give a detailed insight into the physical properties of these novel structures. The approaches used for the analysis and determination of the structure characteristics are based on optical measurements of the exciton states. By use of various measuring techniques like photoluminescence (time-integrated and time-resolved) as well as spin-flip Raman scattering spectroscopy different aspects of the sample characteristics have been determined. In this context the first analytical chapter concentrates on the basic optical properties of the indirect excitons in (In,Al)As/AlAs QDs and allows to assign the observed signals to specific energy levels. Geometrical QD characteristics are here shown to have fundamental influence on both exciton energy and the excitonic recombination times, which is shown to be elevated to the micro- and even millisecond timescales. The spin states defining the fine structure of the excitons are spotlighted in the second analytical chapter which is devoted to the spin-flip Raman scattering spectroscopy. This analytical method is shown to be a powerful tool for the determination of single particle and their complex g-factors. The dynamical behavior of the spin states, i.e. the longitudinal spin relaxation times are investigated in the subsequent chapter in which the spin state population of negatively charged excitons are measured by the degree of circular polarization emission. The population differences leading to the polarization degrees are achieved by both high magnetic fields as well as optical orientation of specific spin states. The mechanism of optical orientation is in the finalizing chapter more systematically studied for specific exciton levels that have been revealed and addressed in the scope of the thesis. In conclusion, it has been shown that indirect excitons in QDs possess spin states that are largely undisturbed by their environment enabling long spin relaxation times and large spin state splitting. The extended exciton recombination times furthermore allow the direct observation of these states by optical means making them highly rewarding structures for scientific research.