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In this study, the effects of thermal annealing on the morphological, structural and optical properties of BGaAs/GaAs Quantum Wells (QWs) are investigated for the first time. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) show that boron islands nucleate on the surface and their rugosity increases after annealing due to boron segregation phenomenon. The segregation is further validated through high-resolution X-ray diffraction (HR-XRD) measurements, which also confirms a good crystalline quality of the system. After annealing at 710 °C for 10 min a significant compositional increment of boron is also observed. Nevertheless, a negligible enhancement of photoluminescence (PL) intensity and minor shift in PL peak energy are found, which is attributed to the defect density and the spatial reduction of localization effects. Time-resolved photoluminescence (TRPL) results exhibit an initial decay time in the microsecond regime from localization emission, followed by an even slower decay from deep levels states. Our results can be the first step towards the engineering of carriers localization in BGaAs/GaAs SQWs by thermal annealing and can orient the material system to be an active layer in intermediate band solar cells.

International audience; InxGa1-xN/GaN solar cells are ideal candidates for use in extreme temperature applications. The conversion efficiency potential of double heterostructure solar cells was investigated at high temperatures using physical simulation. For a targeted working temperature, optimized efficiency lies in a compromise between the absorber bandgap energy determined by In composition and the band offsets at the heterointerface directly correlated with the capability for the photogenerated carriers to cross through the barrier by thermoionic emission. An optimized efficiency of 18% is obtained for an In content of 50% at 400K and decrease down to 10% for an In content of 35% at 500K. As the operating temperature goes higher, the indium content needs to be reduced in order to limit the detrimental effect of increasing intrinsic carrier concentration. The consequence is a decreasing efficiency due to the reduced covered range of the solar spectrum. In the same time, the band offsets are no more a limiting parameter, as there are reduced as the In content decreases, and as higher temperature increases the thermionic transport probability. This result shows the interest of InxGa1-xN/GaN double heterostructure design for high temperatures applications.

The purpose of this article is to study the photocatalytic properties of doped titanate nanotubes prepared by hydrothermal treatment using different metals: Ce, Fe, Zn and Zr at 2 wt% content. The textural and morphological properties of the nanopowders were determined by different analysis techniques such as: XRD, RD/UV–Vis, FTIR and TG-DTA. The results of the characterizations showed that nanotubes can be obtained with a hollow structure, and an important thermal stability of whatever element nature. However, the photocatalytic activity ends with the photodegradation under UV-Visible irradiation of the three industrial molecules: Malachite Green, Methyl Orange and Tannic Acid having different molecule charges. It was noted that the degradation rates of the order of 98% were obtained in fairly short times. The final results obtained revealed that the dopant nature and the solution pH play a major role in determining the photocatalytic degradation of pollutants using TiNTs. However the application of TiNTs and doped TiNTs show that these materials have an inhibitory power of fungi and bacteria quite important.

BGaAs/GaAs epilayers and BInGaAs/GaAs quantum well (QW) have been prepared using metal-organic chemical vapor deposition under different growth conditions, and their physical and structural properties have been examined. SEM-EDS investigation showed a dependence of surface properties of the ternary compound on the growth conditions. High-resolution X-ray diffraction evidenced a tensile strain for the ternary alloys whatever the growth condition, while the quaternary QW always shows a compressive strain state. Room temperature optical absorption allowed to follow the variation of the bandgap with boron incorporation. Photoluminescence measurements confirmed the carrier-localization phenomenon and its dependence with the growth conditions. Deposition temperature and diborane (B2H6) flow rate are with particularly significant effects on the optical properties: lower diborane flow rate and high growth temperature enhance the radiative emission. Computer simulation using localized state ensemble model quantitatively relates the lattice inhomogeneity to the optical properties and suggests a way to engineer the localization phenomenon and avoid clustering effects. Electrical investigations by current-voltage, capacitance and conductance methods have been performed for the first time on selected BGaAs samples. The ideality factor of Schottky barriers has been determined, while their height and film doping level could only be approximately estimated. Such physical properties make boron-based alloys very promising for applications in multijunction solar cells.

The recombination dynamics in BxGa1-xAs alloys on GaAs (001) grown by metal-organic chemical vapor (MOCVD) are investigated using steady-state photoluminescence and Time-Resolved Photoluminescence (TRPL) measurements in the nanosecond range. The decay time as a function of the emission energy and the geometry of the sample (quantum well, epilayer) and post-growth treatment (annealing) is slower for lower-energy emission. In correlation with a systematic study on the photoluminescence features, we have studied the temperature effect on the decay time of the photogenerated carrier. Depending on the geometry of the sample, the exciton dynamics is assisted by the localization phenomenon in the boron-based ternary alloys. It is shown that, at low temperatures, the transfer is mainly ensured by tunneling through localized states where the recombination process is found to be purely radiative in nature. At intermediate temperatures, a competitive process between radiative and non-radiative recombination takes place. For high temperatures, the transfer is assisted by thermal activation, leading to purely non-radiative recombination. We have shown that the unknown state-of-the-art kinetic parameters of the e stage of carrier relaxation and additional carrier recombination dynamics, divulges the particular role of bandgap inhomogeneity and the below-band gap states for the relaxation dynamics that can contribute to the photogeneration mechanisms in BxGa1-xAs/GaAs alloys. Indeed the research findings of this article will help to gain insight into the preliminary carrier relaxation dynamics in boron-based materials.

In this paper, we propose an investigation on the graphene monolayer effect, elaborated on Silicon (Si) by atmospheric-pressure chemical vapor deposition (APCVD), on the carrier localization in ternary InGaN nanorods (NRs) grown by metal-organic chemical vapor deposition (MOCVD). The growth has been proved using scanning electron microscopy (SEM) and Raman technic. SEM investigations support NRs size fluctuations and the presence of random coalescence zones. Raman study reveals the graphene self-doping and low tensile strain which constitutes an origin of the random potential fluctuation. The temperature dependent photoluminescence (PL) of InGaN NRs showed the presence of the localization phenomenon due, principally, to the NRs density and size inhomogeneity and composition fluctuations. Carrier localization increases Auger recombination rates more than radiative rates and is therefore detrimental to the internal quantum efficiency (IQE) of nitrides-based emitters. Our work suggests a partial suppression of the phenomenon effects through a partial suppression of random potential fluctuations by the mean of the direct growth of nitride on Graphene-covered Si substrate. Our qualitative investigations show a reduction of the localization effects due the added graphene monolayer which is quantitatively reinterpreted using the Localized State Ensemble model (LSE) for the first time. Our findings are substantial to advance the integration of nitrides-based devices on any substrates of choice, thereby permitting novel designs of nitrides-based heterojunction device concepts.

Lanthanum-substituted strontium-oxybritholites, Sr10−xLax(PO4)6−x(SiO4)xO with x = 0, 2 and 4, prepared by solid state reaction were investigated by chemical analysis, powder X-ray diffraction, Raman and 29Si MAS NMR spectroscopies. The refinements of powder XRD patterns of the substituted compounds by the Rietveld method showed that the lanthanum occupied the two metal sites, i.e. (4f) and (6h) sites into the apatite structure, with a clear preference for the (6h) sites. A progressive shift of the free oxygen O(4) towards the centre of the triangles formed by the metal-atoms in the (6h) positions was observed when the lanthanum content increased. It led to the formation of a Sr/La(2)-O(4) strong bond, which might have increased the stability of these compounds. The bands of Raman spectra were assigned to the vibration modes of PO4 and SiO4 groups. The comparison of the results of 29Si MAS NMR analysis with those obtained with the 31P previously reported, suggested that both species occupied the same crystallographic sites.

Des fluorapatites substituees au magnesium de formule Ca 10 - x Mg x (PO 4 ) 6 F 2 avec 0≤x≤6 ont ete preparees par la methode hydrothermale. Les echantillons ont ete caracterises par analyse chimique, diffraction des rayons X, spectroscopie infrarouge et mesures de la surface specifique. Pour x