R. Seguin

Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from to as much as with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.

Multi-excitonic complexes in single InGaN quantum dots

Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metalorganic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.

GaN/AlN quantum dots for single qubit emitters

We study theoretically the electronic properties of c-plane GaN/AlN quantum dots (QDs) with the focus on their potential as sources of single polarized photons for future quantum communication systems. Within the framework of eight-band theory we calculate the optical interband transitions of the QDs and their polarization properties. We show that an anisotropy of the QD confinement potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to a pronounced linear polarization of the ground-state and excited-state transitions. An externally applied uniaxial stress can be used to either induce a linear polarization of the ground-state transition for emission of single polarized photons or even to compensate the polarization induced by the structural elongation.

Decay dynamics of neutral and charged excitonic complexes in single InAs∕GaAs quantum dots

Systematic time-resolved measurements on neutral and charged excitonic complexes (X, XX, X+, and XX+) of 26 different single InAs∕GaAs quantum dots are reported. The ratios of the decay times are discussed in terms of the number of transition channels determined by the excitonic fine structure and a specific transition time for each channel. The measured ratio for the neutral complexes is 1.7 deviating from the theoretically predicted value of 2. A ratio of 1.5 for the positively charged exciton and biexciton decay time is predicted and exactly matched by the measured ratio indicating identical specific transition times for the transition channels involved.

Polarized emission lines from single InGaN/GaN quantum dots: Role of the valence-band structure of wurtzite Group-III nitrides

We present a study of the polarization properties of emission lines from single InGaN/GaN quantum dots (QDs). The QDs, formed by spinodal decomposition within ultra-thin InGaN quantum wells, are investigated using single-QD cathodoluminescence (CL). The emission lines exhibit a systematic linear polarization in the orthogonal crystal directions [112¯0] and [1¯100]—a symmetry that is non-native to hexagonal crystals. Eight-band k·p calculations reveal a mechanism that can explain the observed polarizations: the character of the hole(s) in an excitonic complex determines the polarization direction of the respective emission if the QD is slightly elongated. Transitions involving A-band holes are polarized parallel to the elongation; transitions involving B-type holes are polarized in the orthogonal direction. The energetic separation of both hole states is smaller than 10 meV. The mechanism leading to the linear polarizations is not restricted to InGaN QDs, but should occur in other wurtzite-nitride QDs and in materials with similar valence band structure.

Size-Tunable Exchange Interaction in InAs/GaAs Quantum Dots

Single epitaxial quantum dots are promising candidates for the realization of quantum information schemes due to their atom-like electronic properties and the ease of integration into optoelectronic devices. Prerequisite for realistic applications is the ability to control the excitonic energies of the dot. A major step in this direction was recently reached by advanced self-organized quantum-dot growth, yielding ensembles of equally shaped InAs/GaAs dots with a multimodal size distribution. The well-defined sizes of spectrally well separated subensembles enable a direct correlation of structural and excitonic properties, representing an ideal model system to unravel the complex interplay of Coulomb interaction and the quantum dot’s confining potential that depends on size, shape, and composition. In this paper we focus on the exciton-biexciton system with emphasis on the excitonic fine-structure splitting. Across the whole range of size variations within our multimodal quantum dot distribution a systematic trend from +520µeV to −80µeV is found for decreasing dot size. To identify the underlying effects calculations of the fine-structure splitting are performed. A systematic variation of the structural and piezoelectric properties of the modeled quantum dots excludes shape anisotropy and tags piezoelectricity as a key parameter controlling the fine-structure splitting in our quantum dots.

In situ area-controlled self-ordering of InAs nanostructures

Real-time control of self-organized growth of InAs nanostructures has been achieved by employing epitaxial stationary shadow masks in a molecular-beam-epitaxy process. The method is based on the surface diffusion of group-III adatoms governed by the group-V surface concentration. Lateral control is achieved by the geometry of the mask and the incidence angles of the molecular beams. We apply the method to self-organized growth of nanoscale InAs quantum structures at the edge of the incidence region of the arsenic beam. The high quality of the in situ fabricated nanostructures is confirmed by bright cathodoluminescence of InAs quantum wire embedded in GaAs barriers.

Phonons and electronic states of ZnO, Al2O3 and Ge in the presence of time reversal symmetry

Using group theoretical techniques we have investigated all single valued representations as well as double valued, these follow from the inclusion of spin, of wurtzite (e.g. ZnO), trigonal (e.g. Al2O3) and cubic (e.g. Ge) structures, with the C6v4, D3d6 and Oh7 space groups, respectively, with regard to the presence or absence of Time Reversal Symmerty (TRS). We have found a number of phonons in wurtzite and trigonal structures to be time reversal degenerate, whereas in the cubic Si, Ge and diamond the vibrational modes are not time reversal degenerate. Electronic band structure also experience extra TRS degeneracy. Therefore, the selections rules for optical radiative transitions need modification.

Correlation of structural and optical properties of self-organized quantum dots

Interaction between strongly localized charge carriers in zero-dimensional systems like quantum dots (QD) depends sensitively on the geometrical roperties of the dots. The recently observed monolayer splitting with eight well resolved peaks (in low excitation photoluminescence (PL)) together with eight-band kp theory as the appropriate tool for modeling electronic and optical properties offers direct spectroscopic access to details of the QD morphology. By this achievement it became possible to link single-dot spectra obtained by cathodoluminescence measurements via the exciton transition energy to structural properties of the probed QD. In view of theory this situation constitutes an ideal starting point to study few-particle interactions for realistic InAs QDs as a function of their structural properties. This is done using the configuration interaction method. The wavefunctions are obtained from eight-band kp calculations of single-particle states including explicitly piezoelectric effects in the confinement potential.