GaAs nanowires were grown by molecular-beam epitaxy on (111)B oriented surfaces, after the deposition of Au nanoparticles. Different growth durations and different growth terminations were tested. After the growth of the nanowires, the structure and the composition of the metallic particles were analyzed by transmission electron microscopy and energy dispersive x-ray spectroscopy. We identified three different metallic compounds: the hexagonal β′Au7Ga2 structure, the orthorhombic AuGa structure, and an almost pure Au face centered cubic structure. We explain how these different solid phases are related to the growth history of the samples. It is concluded that during the wire growth, the metallic particles are liquid, in agreement with the generally accepted vapor-liquid-solid mechanism. In addition, the analysis of the wire morphology indicates that Ga adatoms migrate along the wire sidewalls with a mean length of about 3μm.

International audienceBroadband light trapping is numerically demonstrated in ultra-thin solar cells composed of a flat amorphous silicon absorber layer deposited on a silver mirror. A one-dimensional silver array is used to enhance light absorption in the visible spectral range with low polarization and angle dependencies. In addition, the metallic nanowires play the role of transparent electrodes. We predict a short-circuit current density of 14:6mA=cm2 for a solar cell with a 90 nm-thick amorphous silicon absorber layer

Magneto-transport experiments have been performed on Quantum Cascade Detectors. These experiments lead to the identification of the different electronic transitions from subbands in one cascade period to subbands in the following one. These transitions contribute to the total current flowing through the structure in the absence of illumination. This dark current is well described within a simple model based on the sum of diffusion events from one cascade to the next one through optical phonon mediated transitions. For the first time, the optical and electronic properties of such a complex heterostructure can be fully predicted without any other adjustable parameter than the doping density. This opens the way to a full quantum design of an infrared detector, in contrast with the phenomenological optimization of structures usually performed in this field.

International audienceWe report high concentration experiments on polycrystalline thin film solar cells. High level regime is reached, thanks to the micrometric scale of the Cu(In,Ga)Se 2 cells, which strongly decreases resistive losses. A 4% absolute efficiency increase is obtained at a concentration of x120, and current densities as high as 100 A/cm² can be measured. These results show that the use of polycrystalline thin films under high concentration is possible, with important technological consequences

No abstract

A pulsed metal-organic chemical vapor deposition technique is developed for the growth of high-quality AlN/GaN superlattices ͑SLs͒ with intersubband ͑ISB͒ transitions at optical communications wavelengths. Tunability of the AlN and GaN layers is demonstrated. Indium is shown to improve SL surface and structural quality. Capping thickness is shown to be crucial for ISB transition characteristics. Effects of barrier-and well-doping on the ISB absorption are reported.

Intersubband ͑ISB͒ absorption at wavelengths as long as 5.3 m is realized in GaN/ Al 0.2 Ga 0.8 N superlattices grown by metalorganic chemical vapor deposition. By employing low aluminum content Al 0.2 Ga 0.8 N barriers and varying the well width from 2.6 to 5.1 nm, ISB absorption has been tuned from 4.5 to 5.3 m. Theoretical ISB absorption and interband emission models are developed and compared to the experimental results. The effects of band offsets and the piezoelectric fields on these superlattices are investigated.

We explore a regime of unipolar electronic transport in a multiple quantum well structure with very large current discontinuities -up to five orders of magnitude. Magnetotransport experiments reveal different transport regimes. Quantum well impact ionization shifts the structure from a resistive "down" state, where the current flows through interwell quantum tunneling, to a highly conductive "up" state. In the latter regime, the current leaks through a barrier suddenly broken down because of an efficient ionization of the first quantum well.