Correlated photon emission from pyramidal quantum dot heterostructures

Quantum dots (QDs) are nanoscopic semiconductor crystals. Electronic transitions inside QDs lead to photon emission with unique properties. Photons can for instance be emitted as triggered single photons or as pairs of correlated photons with different energies. The interest in single and correlated photon emission from semiconductor QDs is mainly motivated by a number of interesting and partly realized application concepts in quantum cryptography and quantum computation. Currently, the most common techniques for fabricating QD single and correlated photon emitters yield QDs with poor control over the QD positions on the fabrication substrate. An alternative fabrication technique yielding perfectly site-controlled QDs inside pyramidal QD heterostructures has recently been established by our group. The objective of the present thesis project was to demonstrate and explore single and correlated photon emission from pyramidal QDs. The motivation for this study was based on the inherent advantage of perfect site-control in the pyramidal QD system, but also in perspective of exploring additional QD properties being important in the context of single and correlated photon emission. Here, particular focus was set on specific optical and electrical pumping schemes, tuning of emission energies, operation at elevated temperatures, and the evaluation of the QD uniformity. All of these aspects are interesting from a fundamental physics point of view, but also with respect to the practical implementation of pyramidal QDs as single photon emitters in future opto-electronic devices. The most important experimental achievement in this thesis project is the first realization of single and correlated photon emission from site-controlled pyramidal QDs under both optical and electrical excitation. The complete results can be summarized as follows: The tunability of the pyramidal QD emission energy was extended towards lower energies from originally 1.55 eV down to 1.43 eV by introducing Indium as a new constituent of the QD material. Moreover, the inhomogeneous broadening of the QD ground state emission energy was determined to be less than 8 meV for a ground-to-excited-state separation of 55 meV. Good reproducibility of the spectral features related to the radiative decay of different neutral and charged excitons was demonstrated by statistical single QD spectroscopy. In this context, the positively charged QD exciton was spectrally identified. All these aspects are beneficial for the flexible and reproducible implementation of single photon emitters. In parallel, QD light emitting diodes were developed, resulting in the first demonstration of pyramidal QD electroluminescence. Specific achievements were, first, the proof of preferential current injection into the QD, which opens the way for e±cient electrical pumping schemes and, second, the realization of an electrically pumped QD single photon source. QD single photon emission under continuous wave optical excitation was demonstrated for sample temperatures up to 80 K. Moreover, resonant optical excitation in pulsed mode resulted in clean triggered single photon emission. In addition, single photon emission from excitons localized in pyramidal QWRs was demonstrated and studied. Finally, photon correlation spectroscopy combined with modelling was used to elucidate formation mechanisms and properties of neutral and charged excitons in pyramidal QDs. The interest in this is for instance based on the potential use of single negatively charged QDs as sources of single photons with enhanced indistinguishability. To summarize, the present thesis has established the pyramidal QD system as a promising candidate for the future implementation of practical single and correlated emitters. In this context, various aspects, which are also interesting from a fundamental physics point of view, were investigated and clarified.