Nonpolar InGaN/GaN multiple quantum wells (MQWs) grown on the {11-00} sidewalls of c-axis GaN wires have been grown by organometallic vapor phase epitaxy on c-sapphire substrates. The structural properties of single wires are studied in detail by scanning transmission electron microscopy and in a more original way by secondary ion mass spectroscopy to quantify defects, thickness (1-8 nm) and In-composition in the wells (∼16%). The core-shell MQW light emission characteristics (390-420 nm at 5 K) were investigated by cathodo- and photoluminescence demonstrating the absence of the quantum Stark effect as expected due to the nonpolar orientation. Finally, these radial nonpolar quantum wells were used in room-temperature single-wire electroluminescent devices emitting at 392 nm by exploiting sidewall emission.

We demonstrate large area fully flexible blue LEDs based on core/shell InGaN/GaN nanowires grown by MOCVD. The fabrication relies on polymer encapsulation, nanowire lift-off and contacting using silver nanowire transparent electrodes. The LEDs exhibit rectifying behavior with a light-up voltage around 3 V. The devices show no electroluminescence degradation neither under multiple bending down to 3 mm curvature radius nor in time for more than one month storage in ambient conditions without any protecting encapsulation. Fully transparent flexible LEDs with high optical transmittance are also fabricated. Finally, a two-color flexible LED emitting in the green and blue spectral ranges is demonstrated combining two layers of InGaN/GaN nanowires with different In contents.

Spectral diffusion is a result of random spectral jumps of a narrow line as a result of a fluctuating environment. It is an important issue in spectroscopy, because the observed spectral broadening prevents access to the intrinsic line properties. However, its characteristic parameters provide local information on the environment of a light emitter embedded in a solid matrix, or moving within a fluid, leading to numerous applications in physics and biology. We present a new experimental technique for measuring spectral diffusion based on photon correlations within a spectral line. Autocorrelation on half of the line and cross-correlation between the two halves give a quantitative value of the spectral diffusion time, with a resolution only limited by the correlation set-up. We have measured spectral diffusion of the photoluminescence of a single light emitter with a time resolution of 90 ps, exceeding by four orders of magnitude the best resolution reported to date

The growth and structural properties of GaN/AlN core-shell nanowire heterostructures have been studied using a combination of resonant x-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy experiments. For a GaN core of 20 nm diameter on average surrounded by a homogeneous AlN shell, the built-in strain in GaN is found to agree with theoretical calculations performed using a valence force field model. It is then concluded that for an AlN thickness up to at least 12 nm both core and shell are in elastic equilibrium. However, in the case of an inhomogeneous growth of the AlN shell caused by the presence of steps on the sides of the GaN core, plastic relaxation is found to occur. Consistent with the presence of dislocations at the GaN/AlN interface, it is proposed that this plastic relaxation, especially efficient for AlN shell thickness above 3 nm, is promoted by the shear strain induced by the AlN inhomogeneity.

The structural and optical properties of InGaN/GaN nanowire heterostructures grown by plasma-assisted molecular beam epitaxy have been studied using a combination of transmission electron microscopy, electron tomography and photoluminescence spectroscopy. It is found that, depending on In content, the strain relaxation of InGaN may be elastic or plastic. Elastic relaxation results in a pronounced radial In content gradient. Plastic relaxation is associated with the formation of misfit dislocations at the InGaN/GaN interface or with cracks in the InGaN nanowire section. In all cases, a GaN shell was formed around the InGaN core, which is assigned to differences in In and Ga diffusion mean free paths.

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We report on the growth of AlxGa1-xN nanowires by plasma-assisted molecular beam epitaxy for x in the 0.3-0.8 range. Based on a combination of macro- and micro-photoluminescence, Raman spectroscopy, x-ray diffraction and scanning electron microscopy experiments, it is shown that the structural and optical properties of AlGaN NWs are governed by the presence of compositional fluctuations associated with strongly localized electronic states. A growth model is proposed, which suggests that, depending on growth temperature and metal adatom density, macroscopic composition fluctuations are mostly of kinetic origin and are directly related to the nucleation of the AlGaN nanowire section on top of the GaN nanowire base which is used as a substrate.

We present a high-temperature single-photon source based on a quantum dot inside a nanowire. The nanowires were grown by molecular beam epitaxy in the vapor-liquid-solid growth mode. We utilize a two-step process that allows a thin, defect-free ZnSe nanowire to grow on top of a broader, cone-shaped nanowire. Quantum dots are formed by incorporating a narrow zone of CdSe into the nanowire. We observe intense and highly polarized photoluminescence even from a single emitter. Efficient photon antibunching is observed up to 220 K, while conserving a normalized antibunching dip of at most 36%. This is the highest reported temperature for single-photon emission from a nonblinking quantum-dot source and principally allows compact and cheap operation by using Peltier cooling.