Electronic Fine Structure and Recombination Dynamics in Single InAs Quantum Dots

Abstract

The electronic structure of semiconductor quantum dots (QDs) is essentially governed by quantum mechanics and differs significantly from the one of bulk semiconductors. This opens possibilities for novel applications based on single QDs like single-photon emitters for quantum cryptography or qubit registers for quantum computing. These aims can only be achieved through a profound understanding and control of the QDs’ electronic properties. These properties strongly depend on the structural parameters of the QDs, such as size, composition, shape, and symmetry. In principle, this allows precise engineering of the electronic characteristics by targeted manipulation of the QDs structure. In the work at hand single InAs/GaAs quantum dots are examined via cathodoluminescence spectroscopy. Isolation of spectra of single QDs from an ensemble is achieved either by appropriate growth of QD samples with low densities or through the application of near-field shadow masks applied on top of the sample surface. The charge carriers confined in the QDs form excitonic complexes, such as neutral and charged excitons and biexcitons, which result in spectrally sharp emission lines upon decay. The lines of different complexes occur at varying energies due to the diverse Coulomb interaction terms between the constituting charge carriers resulting in complex single-QD spectra that frequently consist of up to ten emission lines. A thorough analysis of the spectra, however, leads to an unambiguous assignment of the lines to the decay of specific excitonic complexes. A special aspect of the Coulomb interaction, the exchange interaction, gives rise to a fine structure in the initial and final states of an excitonic decay. This leads to a fine structure in the emission spectra that again is unique for every excitonic complex. One complex can thus show a number of emission lines with different polarization characteristics. The exchange interaction is discussed in great detail in this work. Theoretical considerations help to qualitatively understand the complicated emission spectra emerging from the fine structure. Systematic investigation of spectra of different complexes reveals similarities that originate from the same underlying physical processes thus generating a deep understanding of the exchange interaction between charge carriers in QDs. The structural properties of the QDs have a decisive influence on their electronic structure and fine structure. Here, QDs of different sizes are investigated and the influence on the electronic properties is monitored. Additionally, the structure is modified ex situ by a thermal annealing process. The changes of the spectra under different annealing temperatures are traced. Thus, a possibility for targeted engineering of QDs for applications is demonstrated.