High Curie temperature for La0.7Sr0.3MnO3 thin films deposited on CeO2/YSZ-based buffered silicon substrates

International audienceTwo kinds of epitaxial structures were grown by standard pulsed laser deposition on (001) Si, namely La0.7Sr0.3MnO3/Bi4Ti3O12/CeO2/YSZ/Si (BTO-based), and La0.7Sr0.3MnO3/SrTiO3/ CeO2 /YSZ/Si (STO-based) multilayers. The samples were investigated by means of x-ray diffraction, transmission electron microscopy, magnetic and transport measurements. The Curie temperature TC of the BTO-based samples was found to be higher (360 K) than for the typical reference epitaxial LSMO film grown on (001) SrTiO3 single crystal (345 K), due to high compressive in-plane strain. The STO-based samples show high structural quality, low roughness and high TC (350 K), making them interesting candidates for use in innovative LSMO-based bolometers or spintronic devices operating at room temperature. (Some figures in this article are in colour only in the electronic version

Atomic core structures associated to the threading dislocation with ⟨11ˉ00⟩\langle1\bar 100\rangle Burgers vector and [0001] line direction in GaN

International audienceThe $\langle1\bar 100\rangle$ edge dislocation in GaN has been investigated by means of atomistic computer simulation. It's core structure exhibits three configurations that are related to the location of the origin of displacements imposed to the atoms when generating the dislocation in the initial configuration. They are described by: (a) 8- and 4-coordinated channels (8/4 core); (b) 8-, 5-, and 5-coordinated channels (8/5/5 core); and (c) 9- and 5-coordinated channels (9/5 core). An additional configuration where the $\langle1\bar 100\rangle$ dislocation decomposes in two 8-coordinated channels a= 1/3$\langle1\bar 210\rangle$ dislocations is also generated and it exhibits the lowest energy. This is in agreement with high resolution transmission electron microscopy observations and image simulations carried out on an asymmetric Σ7 [0001] tilt grain boundary in which only cores of elemental a type dislocations have been determined

Strain relaxation in semiconductor heteroepitaxial layers

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Molecular beam epitaxy of semipolar AlN(1122) and GaN(1122) on m-sapphire

International audienceWe report on the plasma-assisted molecularbeam epitaxy of semipolar AlN(112¯2) and GaN(112¯2) films on (11¯00) m-plane sapphire. AlN deposited on m-sapphire settles into two main crystalline orientation domains, AlN(112¯2) and AlN(101¯0), whose ratio depends on the III/ V ratio. Growth under moderate nitrogen-rich conditions enables to isolate the (112¯2) orientation. The in-plane epitaxial relationships of AlN(112¯2) on m-plane sapphire are [112¯3¯]AlN║[0001]sapphire and [11¯00]AlN║[112¯0]sapphire. GaN deposited directly on m-sapphire results in (112¯2)- oriented layers with (101¯3¯)-oriented inclusions. A ~100 nm-thick AlN(112¯2) buffer imposes the (112¯2)-orientation for the GaN layer grown on top. By studying the Ga-desorption on GaN(112¯2), we conclude that these optimal growth conditions corresponds to a Ga excess of one monolayer on the GaN(112¯2) surface

Efficient blocking of planar defects by prismatic stacking faults in semipolar (112¯2)-GaN layers on m-sapphire by epitaxial lateral overgrowth

International audienceFor the next-generation solid state lighting, the production of high quality semipolar (112¯2) GaN layers on sapphire obtained using asymmetric epitaxial lateral overgrowth (ELO) method has been investigated. This type of ELO leads to efficient blocking of the basal stacking faults (BSFs) in the bulk, and enables the formation of nondefective layers at the surface. The BSFs terminate due to generation of prismatic stacking faults along a well defined boundary. The corresponding intensity of GaN band edge photoluminescence emission is increased by more than four orders of magnitude in comparison to that from semipolar templates

Ga kinetics in plasma-assisted molecular-beam epitaxy of GaN(112¯2): Effect on the structural and optical properties

International audienceGaN directly deposited on m-sapphire by plasma-assisted molecular-beam epitaxy settles into two main crystalline orientation domains: GaN﴾112¯2﴿ and GaN{101¯3}. The dominant phase is GaN﴾112¯2﴿ with GaN║ sapphire and GaN║ sapphire in-plane epitaxial relationships. Deposition of GaN on top of an AlN﴾112¯2﴿ buffer layer and growth under slightly Ga-rich conditions reduce GaN{101¯3} precipitates below the detection limits. Studies of Ga adsorption demonstrate that it is possible to stabilize up to one Ga monolayer on the GaN﴾112¯2﴿ surface. The presence of this monolayer of Ga excess on the growth front reduces the ﴾112¯2﴿ surface energy and hence minimizes the surface roughness. Photoluminescence from two-dimensional GaN﴾112¯2﴿ layers is dominated by a near-band-edge emission, which is assigned to excitons bound to stacking faults, present with a density around 3x105 cm−1

Linking litter quality to microbial community structure and functioning in an agricultural soil

EAECOLDURGENOSOLInternational audienceAnthropogenic pressures on agricultural soils are known to alter their biodiversity, which may affect the capacity of ecosystems to deliver functions and services according to the insurance hypothesis on diversity – function relationship. Cropping systems is fuelling belowground biota through the quality of litter supplied, which varies in quality from year to year. Therefore we hypothesized that soil biota structure and functioning strongly respond to crop residues quality. Aerial and underground parts of maize plants were used as labile and recalcitrant litter, respectively. A dynamic experiment was performed using soil columns filled with a silty loam agricultural soil (Estrées-Mons, Northern France) in which we incorporated in the 0-5cm layer either maize leaves or maize roots (and a control treatment without litter). We measured regularly soil respiration, litter quality, enzyme activities, microbial biomass, bacterial and fungal community structure (using 16S and 18S pyrosequencing) during the incubation period. Litter quality strongly influenced degradation rates of litter soluble fraction and polysaccharides, leading to faster C mineralization of maize leaves compared to roots. Enzyme efficiency was higher in the recalcitrant litter demonstrating different microbial strategies. After 15 days of decomposition, leaves stimulated the relative proportion of Proteobacteria and Bacteroidetes (classified as copiotrophic bacteria by Fierer et al. 2007). Similarly in the fungal community, Tremellomycetes and Sordariomycetes exhibited the most copiotrophic behavior. General functions such as soluble compounds mineralization rates were better correlated with microbial biomass (R²=0.65) than with community structure. Nevertheless, more specific functions such as complex C forms degradation were related to bacterial community structure. We concluded that the decomposition of complex C forms, having a high energetic cost for microorganisms, strongly impacts the community structure. Thus, ecosystem functioning depends on biota community structure linked to the resources provided to them

Defect structure in heteroepitaxial semipolar (11¯22) (Ga, Al)N

International audienceThe defect structures in semipolar (11¯22)-GaN, AlN layers grown on m-sapphire by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are characterized by transmission electron microscopy. The epitaxial relationships are identified as [10¯10]GaN ǁ [11¯20]sap and [1¯213]GaN ǁ [0001]sap. Defects are identified as mostly partial dislocations, I1-basal and prismatic stacking faults. The density of dislocations is of the order of 5.5 × 109 cm−2. They are Frank–Shockley partial dislocations with b = 1/6〈20¯23〉 (90%), Shockley partial dislocations with b = 1/3〈10¯10〉 (8%) and perfect dislocations of a-type with b = 1/3〈11¯20〉 (2%). This is in contrast with the growth in c- or a-orientations, where the large majority of extended defects consists of perfect dislocations. Upon MBE regrowth of GaN on MOVPE GaN, no additional defects are generated, although the defects in the substrate propagate through the overgrown layer. However, in the case of MBE deposition of AlN on MOVPE GaN, new threading dislocations of the type b = 1/3〈11¯23〉 are generated taking stepped and curved structures along their lines

Extended defects in nitride layers, influence on the quantum wells and quantum dots

International audienceFor almost two decades, extensive research has been carried out on wurtzite nitride semiconductors due to their large direct band gap which offer the possibility to produce optoelectronic devices from the infrared (InN) to the ultraviolet range (GaN and AlN). The reduction of the defect densities and polarisation effects in nitride heterostructures are still challenges for the production of efficient light emitting diodes and lasers past the green range. In this contribution, the structure of the extended defects is reviewed, and their possible interaction with quantum wells and quantum dots is discussed. The presence of threading dislocations is shown to constitute a driving force for clustering. Inside semipolar layers, the glide planes are not available for the spreading of perfect dislocations, this results in the formation of a more complex defect system which we have investigated in the case of (11-22) layers

Soil micro-food web structure and functions exhibit contrasting dynamics according to litter quality

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