In this thesis, results from studies on (In)GaN quantum dots (QDs) are presented, including investigations of the structural, optical and electronic properties. The experimental studies were performed on GaN and InGaN QDs grown by molecular beam epitaxy, taking advantage of the Stranki-Krastanov growth mode for the GaN QD samples and the composition segregation for the InGaN QD samples.
Optical spectroscopy of the (In)GaN QDs was performed with a combination of different experimental techniques, e.g. stationary microphotoluminescence (μPL) and timeresolved μPL. The μPL spectroscopy is suitable for studies of single QDs due to the wellfocused excitation laser spot, and it typically does not require any special sample preparation. The powerful combination of power and polarization dependences was used to distinguish the exciton and the biexciton emissions from other emission lines in the recorded spectra.
The QDs could be observed with random in-plane anisotropy, as determined by the strong linear polarization for single QDs but with different angular orientation from dot to dot. Additionally, these experimental results are in good agreement with the computational results revealing a similar degree of polarization for the exciton and the biexciton emissions. Further, the theory predicts that the discrepancy of the polarization degree is larger between the positive and negative trions in comparison with the exciton and the biexciton. Based on this result, polarization resolved spectroscopy is proposed as a simple tool for the identification of trions and their charge states.
The fine-structure splitting (FSS) and the biexciton binding energy (Ebxx) are essential QD parameters of relevance for the possible generation of quantum entangled photon pairs in a cascade recombination of the biexciton. In general, the Coulomb interaction between the negatively charged electron and the positively charged hole lifts the fourfold degeneracy of the electron and hole pair ground state, forming a set of zero-dimensional exciton states of unequal energies. This Coulomb-induced splitting, referred to as the FSS, results in an electronic fine structure, which is strongly dependent on the symmetry of the exciton wave function. The FSS was in this work resolved and investigated for excitons in InGaN QDs, using polarization-sensitive μPL spectroscopy employed on the cleaved-edge of the samples. As expected, the FSS is found to exhibit identical magnitudes, but with reversed sign for the exciton and the biexciton. For quantum information applications, a vanishing FSS is required, since otherwise the emissions of the polarization-entangled photon pairs in the cascade biexciton recombination will be prohibited.
The biexcitons are found to exhibit both positive and negative binding energies for the investigated QDs. Since a negative binding energy indicates a repulsive Coulomb interaction, such biexcitons (or exciton complexes) cannot exist in structures of higher dimensionality. On the other hand, a biexciton with a negative binding energy can be found in QDs, since the exciton complexes still remain bound due to their three dimensional confinement. Moreover, the biexciton binding energy depends on the dot size, which implies that a careful size control of dots could enable manipulation of the biexciton binding energy. A large Ebxx value enables better and cheaper spectral filtering, in order to purify the single photon emission, while a proposed time reordering scheme relies on zero Ebxx for the generation of entangled photons.
The dynamics of the exciton and the biexciton emissions from InGaN QD were measured by means of time-resolved μPL. The lifetimes of the exciton related emissions are demonstrated to depend on the dot size. Both the exciton and the biexciton emissions reveal mono-exponential decays, with a biexciton lifetime, which is about two times shorter than the exciton lifetime. This implies that the QD is small, with a size comparable to the exciton Bohr radius. The photon generation rates can be manipulated by controlling the QDs size, which in turn can be utilized for generation of single- and entangled-photons on demand, with a potential for applications in e.g. quantum information.
The polarization of the emitted single photons can be manipulated by using a polarizer, but to the prize of photon loss and reduced emission intensity. Alternative methods to control the polarization of the emission light are a manipulation of the dot symmetry statically by its shape or dynamically by an externally applied electric field. Predictions based on performed calculations show that in materials with a small spin-orbit split-off energy (ΔSO), like the III-nitride materials, the polarization degree of the emission is more sensitive to dot asymmetry than in materials with a large value for ΔSO, e.g. the III-arsenide materials. Moreover, for an electric field applied in the 1͞10 and the 11͞2 directions of the zinc-blende lens-shaped QDs grown on the (111) plane, the polarization degree of InN QDs is found to be significantly more, by a factor of ~50 times, sensitive to the electric field than for GaN QDs. This work demonstrates that especially the InN based QD, are suitable for manipulation of the polarization by the direct control of the dot symmetry or by externally applied electric fields.
Linköping: Linköping University Electronic Press, 2013. , 68 p.
2013-02-07, Plank, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)