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Resonant tunnelling through two impurities has been observed in a vertical double-barrier GaAs/AlGaAs tunnelling diode. It manifests itself as a peak in the I-V characteristic near the conductance threshold. Analysis of the position of the current peak, its shape and the shift in magnetic field parallel to the current provides information about the characteristics of the two impurities. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) of tensile AlGaInAs multi-quantum wells (MQWs) for transverse magnetic (TM) 1.3 μm emitting lasers is presented. Al-containing wells have been mostly studied with compressive strain for transverse electric (TE) lasers. In this study, we report on highly tensile-strained AlGaInAs well layers (−0.72 to −1.65%) grown with compressive-strained AlGaInAs barrier layers (0.64%). The good agreement of high-resolution X-ray curves and simulated curves indicates that good crystalline quality and abrupt hetero-interfaces are obtained. An enhanced separation between light hole and heavy hole transitions is clearly observed by room-temperature photoluminescence as the strain increases. From broad-area laser results, it was observed that the strain had a low impact on the laser internal loss, the quantum efficiency and the transparency current density, which was as low as 0.32 A/cm 2 for a 6 QW structure. On the opposite, a doubling of the gain parameter g 0 when the strain increases from −0.72 to −1.65% was clearly observed. This result is associated with a 40% threshold density reduction on 300 μm long lasers. These investigations show that highly tensile-strained layers are very promising for the realisation of high-speed lasers.

Low Temperature-Spectrally Resolved Cathodo-Luminescence (LT-SRCL) and Cathodo-Luminescence Imaging (CLI) have been used to study the effects of high temperature and high current density aging on 980 nm InGaAs/AlGaAs pump lasers. The aged lasers show a characteristic evolution of the luminescence band related to a thin (≅ 25 nm) InGaP etch-stop layer: a blue-shift of several meV is observed, together with a rather strong increase in the luminescence intensity. The blue-shift is related to an increase in the disorder of the InGaP alloy while the strong increase of the intensity may result from a different localization of the injected carriers due to a modification of the potential profile. A possible microscopic mechanism for the disordering of the InGaP layer is discussed, based on the migration of either p-type impurities (Zn, in this case) or excess group III interstitials.

Our work aimed at a global investigation of the lipid metabolism during the induction of resistance in wheat (Triticum aestivum) against powdery mildew (Blumeria graminis f.sp. tritici). More specifically, the effect of salicylic acid, known as playing a key role in the activation of defence reactions against pathogens in plants, has been investigated. After salicylic acid infiltration, accumulation of phosphatidic acid was observed that could be due to the phospholipase C pathway since an up-regulation of a phospholipase C-encoding gene expression as well as an accumulation of diacylglycerol were observed. The phosphatidic acid accumulation could also result from the phospholipase D pathway since a reduction of phosphatidylethanolamine content occurred. The response to salicylic acid at the octadecanoid pathway level was also investigated: both a lipoxygenase-encoding gene expression and lipoxygenase enzymatic activity were induced by salicylic acid simultaneously with a decrease of the linolenic acid content. Finally, a lipid transfer protein-encoding gene expression was also up-regulated upon salicylic acid infiltration. These observations indicate that lipid metabolism could be considered as a marker of elicitation in wheat.

The contribution of elementary excitations in low-dimensional electron gases to resonant inelastic light scattering is found to be determined by interband transitions involving states at specific wave vectors. In modulation-doped GaAs/GaAlAs quantum wells, we detect only the single-particle excitations (SPE) at resonances with electron-hole transitions at the Fermi wave vector, and only plasmons at resonances with zone-center excitons. The plasmon cross section is comparable to the SPE when double electronic resonance is achieved by tuning the plasmon energy to a valence subband separation.

We perform time integrated degenerate four-wave mixing (DFWM) measurements to investigate exciton dephasing in a GaAs/AlAs lateral superlattice (LSL) with a tunable band gap modulation. The excitons are resonantly excited by femtosecond laser pulses. Under optimal conditions of growth layer alignment, the ground state excitons in the LSL are 1D. When the LSL growth is highly tilted, the 1D confining potential vanishes and the LSL effectively becomes a 2D system. The DFWM measurements for the 1D and 2D excitons show that the exciton–exciton scattering strength is significantly reduced in the 1D case. We find that disorder in the LSL cannot fully explain the observed large reduction. It is likely that reduction in dimensionality plays an important role in this.

The electronic conduction processes in the minibands and excited states of semiconductor superlattices are reviewed and discussed. The paper includes an introduction on miniband conduction and hopping conduction between localized states. It focusses subsequently on interminiband transport, both in the context of theoretical approaches and of experimental observations of these effects. Rabi oscillations between interacting Wannier–Stark levels are considered in detail. The role of scattering, and the manifestation of multipeaked velocity-field relationships on carrier and electric field profiles are evidenced. The final part concerns transport above the superlattice barriers, and quasi-ballistic transport in excited superlattice minibands.

We study the magneto-transport in vertical quantum dots boththeoretically and experimentally. We focus our attention onthe non-linear transport regime. We demonstrate that the peakamplitudes in the I–V characteristics show a dramaticallydifferent behaviour as a function of the magnetic field,depending on the value of the angular momentum of the dotstate through which tunnelling occurs. The investigationallows us to probe details of the quantum dot wavefunctionsand to distinguish tunnelling through localised states relatedto impurities or to quantum dots.

The lateral potential of GaAs/AlAs lateral superlattices (LSL) obtained by organised molecular beam epitaxy is modelled by including segregation effects and step array disorder. This model is compared to experimental data of optical spectroscopy and of thermal dependence of the conductivity anisotropy. The LSL periodicity and amplitude are clearly evidenced in optical spectra whereas the lateral interface abruptness shows up in transport experiments.

We give a short review of some growth issues on vicinal surfaces and of the electronic properties in GaAs/ AlAs Lateral Superlattices (LSL) obtained by molecular beam epitaxy. Original physical effects due to the lower dimensionality are observed in optical and in transport experiments. We get a good understanding of these properties which are associated to the strong in-plane modulation at nanometric lateral scale.

We report three distinct anomalies in emission, absorption and linear emission polarization of two-dimensional degenerate electron systems with tunable lateral potential modulation. We show that they occur for convergent as well as singular Fermi edges with a unique threshold energy below the Fermi level. The resulting spectral hole for convergent edges significantly breaks down the determination of the Fermi energy in the one-electron picture from photoluminescence spectra.

We report on new features in the photoluminescence excitation (PLE) spectra and PLE linear polarization spectra of GaAs/AlAs lateral superlattices grown by molecular beam epitaxy (MBE). These lines appear systematically as the tilt angle of the lateral superlattice is varied. They are identified as zone–edge excitonic transitions by comparison between experimental data and detailed numerical calculations of optical transitions including valence-band mixing and tilt effects.

The anisotropy of the low-temperature electrical conductivity of laterally modulated electron gases can be analyzed in a semi-classical approximation, taking into account both the anisotropy of the group velocity which was already considered in previous works and that of the collision time which was ignored so far. By doing so, both components of the conductivity tensor changes with the modulation amplitude, in agreement with our measurements. We predict the possibility of observing, in realistic structures, longitudinal conductivities larger than the Drude conductivity of similar but unmodulated electron gases.

Semiconductor superlattices are ideal systems to test theories involving band structure properties. We investigate here the ``Zener'' breakdown in the miniband structure of GaAs/GaAlAs superlattices, which, in contrast to interband tunneling in bulk semiconductors, exhibits resonant tunneling properties. The results of dc and microwave transport experiments are explained by a quantum sequential ladder descent model, incorporated in an effective medium description of the transport processes.

to 2° towards ) using ex-situ atomic force microscopy (AFM) under dry nitrogen gas. After growth, the statistical distribution of the terrace width and the correlation length along the steps were consistent with a thermodynamic equilibrium model taking into account an AL-2 step–step interaction. These observations are qualitatively similar to Si and metals after thermal annealing but, in addition to elastic interaction, the dipole–dipole contribution has to be considered in order to explain the large value of the coefficient A (AGa=1.5 eVA) deduced for the force coefficient between the gallium terminated steps. Step bunching occurred for highly Si-doped GaAs growth and we discuss the best method for carrying out the measurements in order to determine the step–step interaction.

The AFM topography of GaAs vicinal surfaces shows that the step-step interaction is dominated by an important dipolar term which compensates the low kink energy to give straight ledges at 600°C. The Ga/Al ordering in GaAs/AlAs lateral superlattices is shown to be limited by the step array disorder as well as by the vertical exchange treated in a mean field approximation.

The amplitude of the 1D periodic potential modulation in GaAs/AlAs lateral superlattices (with 10% mean Al content) obtained by organized epitaxy on GaAs (001) vicinal surfaces is only 0.1 of that expected for perfect lateral atomic ordering. We show that this reduction, deduced optically from the spectral shift due to the modulation, is well explained as the product of a factor 0.41 due to the intrinsic step array disorder of the surface by a factor 0.28 due to the vertical Ga/Al atomic exchange during the growth.

Steps and terraces on GaAs (001) vicinal surfaces misoriented towards (111) or (11) are studied after MBE growth for a large range of misorientation angle (0.2° to 2°) using ex situ contact mode AFM. The terrace width distribution and correlation length are well explained by thermodynamical equilibrium of interacting steps. The step-step interaction is much larger than in elemental materials and is dominated by dipole rather than elastic interaction. Step bunching is never observed on clean surfaces for any misorientation. However, for Si-doped layers, the surface disorder increases and formation of macrosteps occurs at very high doping.

We report on three issues concerning the growth of GaAs AlAs lateral superlattices on vicinal surfaces to obtain quantum wires with negligible inter-wire tunnel coupling. We show that the ledge roughness increases with increasing terrace length and decreases for slower growth rates. The tilt angle of lateral superlattices is not the relevant parameter as long as electronic properties are concerned: one has to consider the product of the coverage error by the lateral superlattice thickness. The Ga flux stability is a critical parameter to be controlled. The use of dual-filament cells improves it by a factor two compared to single-filament ones.

The morphology of MBE grown GaAs vicinal surfaces is studied using ex situ AFM. Analysing the statistical distribution of terrace width, we succeed in explaining our measurements either by a thermodynamical equilibrium model taking into account the step by step interaction or by a 2D Monte Carlo simulation of the growth. In the latter case, an anisotropic Schwoebel barrier at the step edge has to be introduced in order to get a good agreement. We argue that growth is the efficient process to get a low disordered step array at the usual time scale and temperature range of MBE. In addition the effect of a GaAs AlAs buffer superlattice, or of pure AlAs layer, or of Si doping in GaAs is also discussed.

We report on the experimental evidence for quantum confinement in the transport properties of quantum dots fabricated by a novel technique. This technique allows us to confine the electron motion only in the quantum dot, leading to 3D-0D-3D resonant tunnelling transport. A theoretical framework has been developed at the same time to investigate 3D-0D-3D transport devices. The theoretical results agree well with the experimental observations, opening the possibility of directly probing the wavefunctions of 0D structures.

The lateral atomic organization of group III atoms (here Ga and Al) by atomic steps on GaAs substrates slightly misoriented with respect to (001) is far from being perfect. Nevertheless low-disordered in-plane potential modulation of large amplitude (10–50 meV) and short periodicity (

In conventional agricultural practices, fungicides are extensively used to control the development of many fungal plant pathogens. However, the reduction of pesticide applications in the field, which is recommended by authorities and approved by public opinion, may lead to a re-emergence of diseases. Alternative strategies have therefore to be developed in order to control fungal infection; one of them is based on plants natural resistance reinforcement caused by elicitors. Our project aims at characterizing mycosubtilin, surfactin and fengycin, three bacterial cyclic non-ribosomal lipopeptides produced by the rhizobacterium Bacillus subtilis, for their ability to induce resistance in wheat against powdery mildew (Blumeria graminis f.sp. tritici). RT-qPCR approach was used to investigate effect of lipopeptides on expression pattern of defence-related genes, such as PR protein-encoding genes or genes involved in plant defence pathways (reactive oxygen species (ROS) metabolism, lipid peroxidation, phenylpropanoid and octadecanoid pathways) in response to fungus and revealed that expression of LOX and PR1 was strongly induced by surfactin at 24h and 18h after inoculation, respectively. Mycosubtilin seemed to be more efficace to induce expression of OXO and Ltp. Surfactin could also induce expression of chitinase encoding genes but neither surfactin nor mycosubtilin could induce the expression of GSTphi and PAL genes in response to the fungus.

The growth of quantum structures with strong in-plane 1D potential modulation (amplitude , the Fermi energy of an electron gas in a selectively doped heterojunction) at short scale (periodicity , its Fermi wavelength) relies on the understanding of both the epitaxy and the electronic properties. Some issues such as the morphology of the step array, the organization of the Ga and Al atoms on the surface, and the effect of the electron gas screening or of the tilt of the lateral modulation on the effective potential, are discussed.

Nanometer resonant tunnelling diodes, with independent control of the electrodes and quantum well dimensionalities, have been fabricated by a novel technique. Liquid-helium temperature current-voltage ( I–V ) characteristics exhibit a set of resonance lines due to the tunnelling of the 3D electrode electrons through the 0D states of the quantum box. Preliminary results on the lineshape and the relative-intensity dependence of the lines on the radius of the box, and the principal quantum number, are discussed.

The photoluminescence spectrum of modulation doped tilted lateral superlattices is shown to exhibit a systematic enhancement at the Fermi edge. This is explained by considering the nature of the lateral potential which localizes the hole in a 1D potential for vertical lateral superlattices or couples the first two electric subbands in titled lateral superlattices.

Ballistic electron emission microscopy (BEEM), a derivation of scanning tunnelling microscopy (STM), is a recently developed technique that is most often used to measure local potential barrier height variations in metal-semiconductor systems (i.e. Schottky barriers). Here, for the first time, the successful application to the study of buried lateral period superlattices, produced by molecular beam epitaxial growth on a vicinal GaAs(001) substrate, is reported. The structure consists of thin Au and GaAs films on top of the buried AlAs/GaAs superlattice and finally a GaAs substrate. Simultaneously obtained STM topographical images of the Au film and BEEM images of the ballistic electrons reaching the GaAs substrate reveal the expected periodic modulation only in the BEEM image. In addition, spectroscopic examination of the potential barrier at various places over the samples reveals a bimodal distribution consistent with a modulation of Al x Ga 1-x As mole fraction perpendicular to the surface steps. More detailed analysis reveals a compositional modulation larger than previously detected in such samples.

A novel nanofabrication technique for the realization of nanometer resonant tunnelling diodes with independent control of the electrodes and quantum well dimensionalities is presented. Liquid helium temperature current-voltage (I-V) characteristics exhibit a set of resonance lines due to the tunnelling of the 3D electrode electrons through the 0D states of the quantum box. Preliminary results on the line shape and on the relative intensity dependence of the lines with the radius of the box and the principal quantum number are also discussed.

Laterally modulated nanostructures, exhibiting large optical and electrical anisotropy, can be grown on vicinal surfaces. However the lateral organization of Ga and Al atoms is far from ideal. We discuss our way of understanding the growth of these structures.

The Sterol Biosynthesis Inhibitor (SBI) fungicide, propiconazole, is extensively used in modern agriculture to control fungal diseases. Unfortunately, little is known about its potential side effects on non-target plant-beneficial soil organisms such as arbuscular mycorrhizal fungi (AMF). The direct impact of increasing propiconazole concentrations (0.02; 0.2 and 2 mg x L(-1)) on the lipid metabolism of the AMF Glomus irregulare in relation with its development, was studied by using axenic cultures. The propiconazole impact on G. irregulare was investigated, firstly, through sterol (the target-metabolism of SBI fungicides), phospholipids (PL) and their associated fatty acids (PLFA) analysis (the main membrane components) and secondly by measuring malondialdehyde (MDA) (a biomarker of lipid peroxidation) formation. Finally, the storage lipid quantity, triacylglycerol (TAG), was quantified. Our results demonstrated that the drastic reduction of G. irregulare development (germination, germ tube elongation, colonization, extraradical hyphae growth and sporulation) could be explained not only by the decreases of the total sterol end-products (24-methylcholesterol and 24-ethylcholesterol) and by 24-methylene dihydrolanosterol (a sterol precursor) accumulation, suggesting an inhibition of a key enzyme in sterol biosynthesis pathway (14alpha-demethylase), but also by the increases in phosphatidylcholine (PC) and PLFA (C16:0; C18:0 and C18:3) quantities as well as by MDA accumulation. Moreover, TAG quantity was found to be reduced in the presence of propiconazole, suggesting their use by G. irregulare in a response to propiconazole toxicity. In conclusion, taken together, the findings of the current study highlighted a relationship between the SBI fungicide toxicity against the beneficial AMF G. irregulare and (1) the disturbance in the sterol metabolism, (2) the membrane alteration (PC decrease, lipid peroxidation) as well as (3) the reduction in storage lipids, TAG. More generally, this work could contribute to investigate the toxicity of agricultural chemicals on AMF and underlined the emergency of using sustainable alternative method to control plant diseases. Furthermore, these data can provide a useful approach in soil ecotoxicology studies and risk assessment.

We first report on a high average output power (>3dBm) 1.55μm EML TOSA able to address industrial environment temperature range [−40/+90°C] with Ultra-Low Power consumption (0.8W) and power penalty

We present investigations of the optical properties of quantum wire arrays grown on vicinal surfaces with lateral periods between 8 nm and 32 nm. Because of the decreasing tunnelling between adjacent wires when the lateral periodicity gets larger, the dimensionality changes from two to one. This has been evidenced, experimentally and theoretically, by a red-shift of the photoluminescence and by the appearance of spectral polarization oscillations which are indicative of one-dimensional subbands.

We have investigated the anisotropy of optical properties in quantum wells grown on vicinal surfaces and modulated by a fractional AlAs layer inserted in their middle. Photoluminescence linear polarization is systematically measured as a function of terrace length and of AlAs terrace coverage. Our results highlight the influence of anisotropic islands on optical properties.

We describe the realization of very high peak to valley current ratio double barrier resonant tunneling structures by molecular beam epitaxy. We find that the resonant current through the ground state resonance decreases as the temperature is raised. This unique behavior is explained within the Esaki-Tsu formalism and originates in the Fermi-Dirac statistics.

We propose an experimental method for the optical detection of transport along the growth axis of GaAs/Al x Ga 1-x As superlattices (SL) that combines stationary photoluminescence excitation experiments (PLE) and time-resolved picosecond spectroscopy. We show that both the absorption coefficient of the structure and the effective diffusion length of the photogenerated excitations can be deduced from PLE experiments performed on structures with two enlarged test wells (EW) grown on both sides of the SL

Low levels of intensity noise in semiconductor lasers is a key feature for numerous applications such as high resolution spectroscopy, fiber-optic sensors, signal distribution in broadband analog communications as CATV, and more generally for microwave photonics systems. In particular, a DFB laser with very low relative intensity noise (RIN) levels from 0.1 to 20 GHz is a key component as it correspond to the whole frequency bandwidth of interest for radars. Several approaches have been reported but most suffer from the compromise between RIN level and power out level and stability, with RIN level in the range -150 dB.Hz -1 to -155 dB.Hz -1 in this frequency range [1,2]. We report here results from a new AlGaInAs DFB laser developed at 3S PHOTONICS. Excellent device performance is observed across an operating range from the laser threshold up to the thermal roll-over. Pure longitudinal single mode at 1545 nm is obtained over the whole current operating range with side mode suppression ratio higher than 50dB. The maximum output power reaches up to 130 mW. In these conditions, RIN levels below -160 dB.Hz -1 is obtained in up to 20 GHz. These are the best results to our knowledge combining such high single mode output power with such low RIN level in the frequency range 0.4-20 GHz.

We investigate the asymmetry of I – V characteristics in a series of double barrier resonant tunneling structures (DBRTS) in which we intentionally insert a Si-delta doping sheet of varied concentration in one of the barrier. I – V characteristics are strongly affected by the presence of ionized impurities. We explain the peak current densities and the peak voltage variations within a coherent tunneling model while the potential profile is known from the solution of Poisson's equation. Our results suggest that the valley current originates from diffusion within the barrier between the accumulation layer and the quantum well (QW). The contribution of the exit barrier (between the QW and the depletion layer) to the valley current is less important.

This work presents a global investigation of total fatty acid (FA) content in wheat in relation to treatment with four inducers of resistance and to powdery mildew infection. Linolenic acid (C18:3), linoleic acid (C18:2) and palmitic acid (16:0) were the most abundant FAs in wheat leaves. We investigated the effect of the following inducers of resistance: Iodus40, heptanoyl salicylic acid (HSA), Milsana and trehalose on FA accumulation. Previous studies established that lipid metabolism is altered by these compounds, and we therefore aimed to characterise their impact at the FA level. During a time course experiment, content (quantitative analysis) and percentage (qualitative analysis) of FAs were compared in treated plants and in controls, as well as in plants inoculated with Blumeria graminis f. sp. tritici (i) and non-inoculated (ni) plants. No change in C18:3 content was observed. C18:1 in Iodus 40-treated (ni) plants showed a quantitative 1.2-fold increase. Lauric acid (C12:0) content quantitatively increased after Iodus 40 (2.8-fold), Milsana (4.8-fold) and trehalose (4.0-fold) treatment in (i) plants. However, eicosadienoic acid (C20:2) quantitatively decreased in (ni) plants after Iodus 40 (1.5-fold) and Milsana (2.3-fold) treatment. The amount of C18:2 increased (1.6-fold) after HSA treatment in (i) plants. All these variations in FA content were correlated with variations in the corresponding relative percentages. Our work provides the first evidence for alterations in C12:0, C18:1, C18:2 and C20:2 FA content caused by four resistance inducers. We also compared the amount and percentage of each FA in untreated (i) and (ni) plants. In (i) plants, eicosadienoic acid (C20:2) increased and C18:2 decreased slightly. The potential involvement of these FAs during induced resistance and infection is discussed.

We perform temperature-dependent photoluminescence measurements on a GaAs/AlAs quantum wire array (QWR-A) with a very low disorder. Using these, we deduce the temperature dependence of the homogeneous linewidth of the quasi-one-dimensional exciton to show that exciton scattering rates with optical phonons are enhanced in the QWR-A in comparison with a reference quantum well.

1550 nm AlGaInAs DFB lasers and PIN receptors were developed for analog transmission up to 20 GHz. Over 20 GHz bandwidth are measured for both laser and detector. Highly linear characteristics and low RIN < -150 dB/Hz have been obtained for lasers.

A high energy focused ion beam (Ga+, 150 keV) is used to implant and interdiffuse quantum well structures. Optical properties of wires and boxes created by this technique are evaluated by low temperature cathodoluminescence spectroscopy and imaging. We propose two different approaches to optimize the interdiffusion induced energy shift. We show the large energy shifts observed in the narrow wires and boxes (below 4000 A) are dominated by straggling effects.

We report on photocurrent spectra of quasiperiodic superlattices which are greatly modified when an electric field is applied along the growth axis. We explain this behavior with a simple tight-binding model including only the superlattices sequences on which the different electronic states are peaked at zero electric field thus confirming the wavefunction “localization” of critical states.

We demonstrate record kink-free output powers, over 1.2 W, for 1060-nm single-mode lasers. Saturation powers of 1.9 W are observed at room temperature that guarantee high-power and kink margins with respect to typical operation conditions.

We present the first results of transmission up to 80°C over 100 km of standard fiber using an integrated electro-absorption modulator lasers. This enables uncooled small form factor metropolitan transmitters.

We report on the development of a new generation of very high power 980 nm single lateral mode ridge-waveguide quantum-well lasers. An asymmetric-waveguides vertical structure has been optimized for very low internal losses while keeping the vertical mode-size large, thus allowing a low vertical far-field beam angle of less than 19°. Careful optimization of the doping profiles, and epitaxial interfaces optimization for reduced scattering, allowed to obtain internal losses as low as 0.6-0.7 cm -1 . Such low losses are necessary to keep the external efficiency high in very long cavities, together with a high internal quantum efficiency. We thus reached our goal of keeping the external efficiency above 70% even for cavity lengths of 4.5 mm. The flared ridge waveguide has been designed to strongly filter higher order lateral modes, and kink-free operation has been obtained up to over 1.5 W output power, with very stable vertical and horizontal beam patterns. High saturation powers above 2 W have also been demonstrated at 25°C, and over 1.5 W at 75°C. Wavelength stabilized chips, by means of a fiber Bragg grating, reached linear fiber powers above 1.0 W with strong suppression of gain-peak lasing at all currents and good power stability.