In this paper we investigate the coherence properties of a quantum dot under two-photon resonant excitation in combination with an additional photo-neutralization laser. The photo-neutralization increases the efficiency of the excitation process and thus, the brightness of the source, by a factor of approximately 1.5 for biexciton-exciton pairs. This enhancement does not degrade the relevant coherences in the system; neither the single photon coherence time, nor the coherence of the excitation process.

We report on nonpolar GaN quantum dots embedded in AlN, grown on (11-20) 6H–SiC by plasma-assisted molecular-beam epitaxy. These dots are aligned in the growth plane and present a constant aspect ratio of 10. Their optical properties were studied as a function of GaN coverage. Especially, the variation of their emission energy as compared to that of (0001) GaN quantum dots is a clear fingerprint of the reduced internal electric field present in these nonpolar nanostructures. Time-resolved spectroscopy confirmed this result by revealing lifetimes in the few 100 ps range in contrast to the much longer ones obtained for the (0001) GaN quantum dots.

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The paper reports on a study of the emission of GaN/AlN self-assembled quantum dots grown on a-plane 6H-SiC showing evidence of the suppression of the internal electric field. The strain in dots and barriers is determined by means of Raman scattering and the induced piezoelectric polarizations are estimated. These reveal a compensation of the spontaneous polarization and justify the lack of a quantum confined Stark effect found in the photoluminescence spectra. Strain effects and strong confinement are responsible for the partial depolarization of the emission and its energy dependence.

The optical orientation of the exciton spin in an ensemble of self-organized cubic GaN / AlN quantum dots is studied by time-resolved photoluminescence. Under a polarized quasiresonant excitation, the luminescence linear polarization exhibits no temporal decay, even at room temperature. This demonstrates the robustness of the exciton spin polarization in these cubic nitride nanostructures, with characteristic decay times longer than 10 ns.

The growth of (11–20) or a-plane quantum dots and quantum wells by plasma-assisted molecular-beam epitaxy has been studied. It is shown that Ga-rich conditions lead to the formation of quantum dots, whereas quantum wells are obtained in N-rich conditions. Combining various experimental techniques, it is furthermore demonstrated that quantum dot nucleation along [1–100] and quantum well morphology in the (1–100) plane are influenced by anisotropic growth of AlN buffer layer. Moreover, it is established that peculiar morphological features of quantum dots and quantum wells, in particular the asymmetric shape of quantum dots, are related to the polar character of the [0001] direction in wurtzite nitride material.

Articles you may be interested inStructural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy Medium energy ion scattering ͑MEIS͒ has been used to measure at the monolayer scale the strain profile of self-organized GaN quantum dots grown on ͑11-20͒ or a-plane AlN by molecular-beam epitaxy. By confronting the MEIS results with a structural analysis carried out by atomic force microscopy, it is established that the strain profile is anisotropic, i.e., fully elastic along ͓1-100͔ and a combination of plastic and elastic along ͓0001͔. High resolution transmission electron microscopy measurements reveal the presence of misfit dislocations with 1/2 ͓0001͔ Burgers vector, consistent with MEIS data.

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