Diversité des pratiques d'élevage bovin à viande dans le massif du Dahra (Algérie)

Pour appréhender la diversité des pratiques d’élevage bovin à viande dans les zones rurales du massif du Dahra (nord-ouest algérien), une typologie a été réalisée à partir des résultats d’une enquête dans 56 exploitations. Les groupes définis diffèrent par les structures et sont de 5 types (A, B, C, D et E). L’analyse technico - économique de 20 exploitations réparties sur les différents types préalablement identifiés révèle 4 systèmes de pratiques différents selon l’objectif de production, le plus souvent en cohérence avec les structures disponibles. Le contrôle des performances réalisé au niveau de 9 exploitations montre que lorsqu'on exclut le système d’élevage bovin commercial sans femelles reproductrices (engraisseur), le système à logique de capitalisation apparaît alors comme le plus performant

InAs/GaAs quantum-dot superluminescent light-emitting diode monolithically grown on a Si substrate

Building optoelectronic devices on a Si platform has been the engine behind the development of Si photonics. In particular, the integration of optical interconnects onto Si substrates allows the fabrication of complex optoelectronic circuits, potentially enabling chip-to-chip and system-to-system optical communications at greatly reduced cost and size relative to hybrid solutions. Although significant effort has been devoted to Si light generation and modulation technologies, efficient and electrically pumped Si light emitters have yet to be demonstrated. In contrast, III–V semiconductor devices offer high efficiency as optical sources. Monolithic integration of III–V on the Si platform would thus be an effective approach for realizing Si-based light sources. Here, we describe the first superluminescent light-emitting diode (SLD) monolithically grown on Si substrates. The fabricated two-section InAs/GaAs quantum-dot (QD) SLD produces a close-to-Gaussian emission spectrum of 114 nm centered at 1255 nm wavelength, with a maximum output power of 2.6 mW at room temperature. This work complements our previous demonstration of an InAs/GaAs QD laser directly grown on a Si platform and paves the way for future monolithic integration of III–V light sources required for Si photonics

Monolithically Integrated InAs/GaAs Quantum Dot Mid-Infrared Photodetectors on Silicon Substrates

High-performance, multispectral, and large-format infrared focal plane arrays are the long-demanded third-generation infrared technique for hyperspectral imaging, infrared spectroscopy, and target identification. A promising solution is to monolithically integrate infrared photodetectors on a silicon platform, which offers not only low-cost but high-resolution focal plane arrays by taking advantage of the well-established Si-based readout integrated circuits. Here, we report the first InAs/GaAs quantum dot (QD) infrared photodetectors monolithically integrated on silicon substrates by molecular beam epitaxy. The III–V photodetectors are directly grown on silicon substrates by using a GaAs buffer, which reduces the threading dislocation density to ∼106 cm–2. The high-quality QDs grown on Si substrates have led to long photocarrier relaxation time and low dark current density. Mid-infrared photodetection up to ∼8 μm is also achieved at 80 K. This work demonstrates that III–V photodetectors can directly be integrated with silicon readout circuitry for realizing large-format focal plane arrays as well as mid-infrared photonics in silicon

Al0.2Ga0.8As solar cells monolithically grown on Si and GaAs by MBE for III-V/Si tandem dual-junction applications

Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on silicon substrates by Molecular Beam Epitaxy. Due to the 4% lattice mismatch between AlGaAs and Si, Threading Dislocations (TDs) nucleate at the III-V/Si interface and propagate to the active region of the cells where they act as recombination centers, reducing the performances of the devices. In order to reduce the Threading Dislocation Density (TDD) in the active layers of the cells, InAlAs Strained Layer Superlattice (SLS) Dislocation Filter Layers (DFLs) have been used. For one of the samples, in-situ Thermal Cycle Annealing (TCA) steps have additionally been performed during growth. For comparison purposes, reference Al0.2Ga0.8As solar cells have been grown lattice-matched on GaAs. For the sample grown on Si without TCA, the TDD has been reduced from over 7×109cm-2 at the III-V/Si interface to 3×107cm-2 in the base of the cells. With TCA, the TDD has been reduced throughout the sample from over 3×109cm-2 in the initial epilayers to 8(±2)×106cm-2 in the base of the cells. For the best devices, the Voc improves from 833mV on Si without TCA to 895mV using TCA, compared with 1070mV for the reference sample grown lattice-matched on GaAs. Similarly the fill factor improves from 73.7% on Si without TCA to 74.8% using TCA, compared with 78.4% on GaAs. The high bandgap-voltage offset obtained both on Si and GaAs indicates a non-optimal bulk AlGaAs material quality due to non-ideal growth conditions

1.7eV Al0.2Ga0.8As solar cells epitaxially grown on silicon by SSMBE using a superlattice and dislocation filters

Lattice-mismatched 1.7eV Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on Si substrates using Solid Source Molecular Beam Epitaxy (SSMBE). As a consequence of the 4%-lattice-mismatch, threading dislocations (TDs) nucleate at the interface between the Si substrate and III-V epilayers and propagate to the active regions of the cell. There they act as recombination centers and degrade the performances of the cell. In our case, direct AlAs/GaAs superlattice growth coupled with InAlAs/AlAs strained layer superlattice (SLS) dislocation filter layers (DFLSs) have been used to reduce the TD density from 1×10^9cm^-2 to 1(±0.2)×10^7cm^-2. Lattice-matched Al0.2Ga0.8As cells have also been grown on GaAs as a reference. The best cell grown on silicon exhibits a Voc of 964mV, compared with a Voc of 1128mV on GaAs. Fill factors of respectively 77.6% and 80.2% have been calculated. Due to the lack of an anti-reflection coating and the non-optimized architecture of the devices, relatively low Jsc have been measured: 7.30mA.cm^-2 on Si and 6.74mA.cm^-2 on GaAs. The difference in short-circuit currents is believed to be caused by a difference of thickness between the samples due to discrepancies in the calibration of the MBE prior to each growth. The bandgap-voltage offset of the cells, defined as Eg/q-Voc, is relatively high on both substrates with 736mV measured on Si versus 572mV on GaAs. The non-negligible TD density partly explains this result on Si. On GaAs, non-ideal growth conditions are possibly responsible for these suboptimal performances

Defect-Free Self-Catalyzed GaAs/GaAsP Nanowire Quantum Dots Grown on Silicon Substrate

The III-V nanowire quantum dots (NWQDs) monolithically grown on silicon substrates, combining the advantages of both one- and zero-dimensional materials, represent one of the most promising technologies for integrating advanced III-V photonic technologies on a silicon microelectronics platform. However, there are great challenges in the fabrication of high-quality III-V NWQDs by a bottom-up approach, that is, growth by the vapor-liquid-solid method, because of the potential contamination caused by external metal catalysts and the various types of interfacial defects introduced by self-catalyzed growth. Here, we report the defect-free self-catalyzed III-V NWQDs, GaAs quantum dots in GaAsP nanowires, on a silicon substrate with pure zinc blende structure for the first time. Well-resolved excitonic emission is observed with a narrow line width. These results pave the way toward on-chip III-V quantum information and photonic devices on silicon platform

On the similarity of Sturm-Liouville operators with non-Hermitian boundary conditions to self-adjoint and normal operators

We consider one-dimensional Schroedinger-type operators in a bounded interval with non-self-adjoint Robin-type boundary conditions. It is well known that such operators are generically conjugate to normal operators via a similarity transformation. Motivated by recent interests in quasi-Hermitian Hamiltonians in quantum mechanics, we study properties of the transformations in detail. We show that they can be expressed as the sum of the identity and an integral Hilbert-Schmidt operator. In the case of parity and time reversal boundary conditions, we establish closed integral-type formulae for the similarity transformations, derive the similar self-adjoint operator and also find the associated "charge conjugation" operator, which plays the role of fundamental symmetry in a Krein-space reformulation of the problem.Comment: 27 page