A systematic variation of the exciton fine-structure splitting with quantum dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from -80 to as much as 520 µeV with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band k·p 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.The exchange interaction of electron-hole pairs (excitons) in semiconductor quantum dots (QDs) has been subject of a lively debate in recent years [1,2,3,4,5,6,7,8]. In such strongly confined systems it is supposed to be enhanced with respect to the bulk case due to the close proximity of electrons and holes. However, the influence of the exact geometry of the confining potential on the exchange interaction still needs to be clarified. A detailed understanding of the resulting exciton fine structure in quantum dots is of fundamental interest and of largest importance for potential applications of QDs in single-photon emitters and entangled two-photon sources for quantum cryptography [9].The total angular momentum M of heavy-hole excitons (X) in QDs is composed of the electron spin (s = ± 1 2 ) and the heavy hole angular momentum (j = ± 3 2 ), consequently producing four degenerate exciton states frequently denoted as dark (M = ±2) and bright (M = ±1) states indicating whether they couple to the photon field or not. Independent of the given confinement symmetry electron-hole exchange interaction causes a dark-bright splitting. Furthermore it mixes the dark states lifting their degeneracy and forming a dark doublet (|2 ±|−2 ). Likewise, additional lowering of the confinement symmetry to C 2v or lower mixes the bright states producing a nondegenerate bright doublet (|1 ± | − 1 ).While emission lines involving pure states are circularly polarized, the mixed states usually produce lines showing linear polarization along the [110] and [110] crystal directions, respectively (Fig. 1). The two bright states are thus directly observable as linearly polarized transitions in luminescence experiments. The energetic difference between these lines is called exciton fine-structure splitting (FSS).The biexciton (XX) ground state is not split by the exchange interaction, since the net spin of the involved electrons and holes is 0. However, the XX to X decay involves two allowed transitions with the final states being the bright states of the X. Therefore, the FSS is reproduced (yet inverted) in the XX to X decay (Fig. 1).Recently reported experimental values of the FSS in * Email: seguin@sol.physik.tu-berlin.de

Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metal organic 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.Comment: 3 pages, 4 figure

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

Eliminating the fine structure splitting of excitons in self-assembled InAs/GaAs quantum dots via combined stresses Appl. Phys. Lett. 101, 063114 (2012); 10.1063/1.4745188 Electrical control of fine-structure splitting in self-assembled quantum dots for entangled photon pair creation Appl. Phys. Lett. 97, 221108 (2010); 10.1063/1.3522655 Exsitu control of finestructure splitting and excitonic binding energies in single InAs/GaAs quantum dots AIP Conf.

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.