GaN/AlN Quantum Dots for Single Qubit Emitters

We study theoretically the electronic properties of $c$-plane GaN/AlN quantum dots (QDs) with focus on their potential as sources of single polarized photons for future quantum communication systems. Within the framework of eight-band k.p 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.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte

Band gap and band parameters of InN and GaN from quasiparticle energy calculations based on exact-exchange density-functional theory

We have studied the electronic structure of InN and GaN employing G0W0 calculations based on exact-exchange density-functional theory. For InN our approach predicts a gap of 0.7 eV. Taking the Burnstein-Moss effect into account, the increase of the apparent quasiparticle gap with increasing electron concentration is in good agreement with the observed blue shift of the experimental optical absorption edge. Moreover, the concentration dependence of the effective mass, which results from the non-parabolicity of the conduction band, agrees well with recent experimental findings. Based on the quasiparticle band structure the parameter set for a 4x4 kp Hamiltonian has been derived.Comment: 3 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

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 [1 1 -2 0] and [-1 1 0 0]--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.Comment: Conf. Proc. of the MSS-13 in Genova 2007, accepted at Physica

Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots

A systematic study of the impact of annealing on the electronic properties of single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different excitonic complexes change dramatically. The results are compared to model calculations within eight-band k.p theory and the configuration interaction method, suggesting a change of electron and hole wave function shape and relative position.Comment: 4 pages, 4 figure

Strain effects and band parameters in MgO, ZnO, and CdO

We have derived consistent sets of band parameters (bandgaps, crystal-field splittings, effective masses, Luttinger, and EP parameters) and strain deformation potentials for MgO, ZnO, and CdO in the wurtzite phase. To overcome the limitations of density-functional theory in the local-density and generalized gradient approximations, we employ a hybrid functional as well as exact-exchange-based quasiparticle energy calculations in the G0W0 approach. We demonstrate that the band and strain parameters derived in this fashion are in very good agreement with the available experimental data and provide predictions for all parameters that have not been determined experimentally so far. VC 2012 American Institute of Physics

Interrelation of structural and electronic properties of InGaN/GaN quantum dots using an eight-band k.p model

We present an eight-band k.p model for the calculation of the electronic structure of wurtzite semiconductor quantum dots (QDs) and its application to indium gallium nitride (InGaN) QDs formed by composition fluctuations in InGaN layers. The eight-band k.p model accounts for strain effects, piezoelectric and pyroelectricity, spin-orbit and crystal field splitting. Exciton binding energies are calculated using the self-consistent Hartree method. Using this model, we studied the electronic properties of InGaN QDs and their dependence on structural properties, i.e., their chemical composition, height, and lateral diameter. We found a dominant influence of the built-in piezoelectric and pyroelectric fields, causing a spatial separation of the bound electron and hole states and a redshift of the exciton transition energies. The single-particle energies as well as the exciton energies depend heavily on the composition and geometry of the QDs

Polarized emission lines from A- and B-type excitonic complexes in single InGaN/GaN quantum dots

Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [1120] and [1100]. Realistic eight-band k·p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are formed either by the A or the B valence band

Singular open book structures from real mappings

We prove extensions of Milnor's theorem for germs with nonisolated singularity and use them to find new classes of genuine real analytic mappings $\psi$ with positive dimensional singular locus \Sing \psi \subset \psi^{-1}(0), for which the Milnor fibration exists and yields an open book structure with singular binding.Comment: more remark

Multiband tight-binding theory of disordered ABC semiconductor quantum dots: Application to the optical properties of alloyed CdZnSe nanocrystals

Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type ABC can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated from a multiband sp^3 empirical tight-binding model in combination with the configuration interaction scheme to calculate the optical properties of CdZnSe nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation (VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and concentration. We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient for further sizes

Linearly polarized photoluminescence of InGaN quantum disks embedded in GaN nanorods

We have investigated the emission from InGaN/GaN quantum disks grown on the tip of GaN nanorods. The emission at 3.21 eV from the InGaN quantum disk doesn't show a Stark shift, and it is linearly polarized when excited perpendicular to the growth direction. The degree of linear polarization is about 39.3% due to the anisotropy of the nanostructures. In order to characterize a single nanostructure, the quantum disks were dispersed on a SiO2 substrate patterned with a metal reference grid. By rotating the excitation polarization angle from parallel to perpendicular relative to the nanorods, the variation of overall PL for the 3.21 eV peak was recorded and it clearly showed the degree of linear polarization (DLP) of 51.5%