117 research outputs found
Etude structurale des verres de borates par une approche combinant résonance magnétique nucléaire du solide et dynamique moléculaire
International audienceL'oxyde de bore B O est ajouté à de nombreuses compositions verriÚres dans le but d'améliorer certaines propriétés, comme dans le verre Pyrex ou Duran. Pour étudier la structure et les propriétés des verres, la Résonance Magnétique Nucléaire est de plus en plus utilisée. Il est également fréquent de réaliser des simulations numériques des structures étudiées afin d'aider à l'analyse des données RMN obtenues expérimentalement. La possibilité de réaliser des calculs de paramÚtres RMN sur des structures de quelques centaines d'atomes par des méthodes DFT permet de faire le lien entre RMN et DM. Dans cette étude, des verres de compositions xNaO-(1-x)BO ont été étudiés. En parallÚle des mesures RMN à champs multiples réalisées (7.0, 9.4, 11.75 et 18.8T), des simulations par dynamique moléculaires (DM) ont été menées sur des systÚmes de 100-200 atomes, soit en utilisant des potentiels classiques soit par des calculs ab-initio (AIMD), suivies de calculs DFT des paramÚtres RMN par la méthode GIPAW
k-space spin filtering effect in the epitaxial Fe/Au/Fe/GaAs(001) spin-valve
International audienceThe hot-electron magnetotransport of epitaxial Fe/Au/Fe/GaAs(001) spin-valves is investigated by ballistic-electron magnetic microscopy. A magnetocurrent amplitude larger than 500% is observed at room temperature close to the Schottky barrier energy. Remarkably, this magnetocurrent is not significantly affected by the thickness reduction of ferromagnetic films, down to 5 atomic layers of the Fe(001) top electrode. This rather suggests a dominant interfacial spin-filtering effect. Finally, the magnetocurrent is strongly reduced when the effective mass of the semiconductor collector is increased. These observations are consistent with recent theoretical prediction of k-space spin-filtering effect in epitaxial spin-valves attached to a semiconducting lead
Epitaxial ZnO thin films grown by pulsed electron beam deposition
In this work, the pulsed electron beam deposition method (PED) is evaluated by studying the properties of ZnO thin films grown on c-cut sapphire substrates. The film composition, structure and surface morphology were investigated by means of Rutherford backscattering spectrometry, X-ray diffraction and atomic force microscopy. Optical absorption, resistivity and Hall effect measurements were performed in order to obtain the optical and electronic properties of the ZnO films. By a fine tuning of the deposition conditions, smooth, dense, stoichiometric and textured hexagonal ZnO films were epitaxially grown on (0001) sapphire at 700 °C with a 30° rotation of the ZnO basal plane with respect to the sapphire substrate. The average transmittance of the films reaches 90% in the visible range with an optical band gap of 3.28 eV. Electrical characterization reveals a high density of charge carrier of 3.4 à 10 19 cm -3 along with a mobility of 11.53 cm2/Vs. The electrical and optical properties are discussed and compared to ZnO thin films prepared by the similar and most well-known pulsed laser deposition method
Theoretical and experimental study of (In,Ga)As/GaP quantum dots
International audience(In,Ga)As/GaP(001) quantum dots (QDs) are grown by molecular beam epitaxy and studied both theoretically and experimentally. The electronic band structure is simulated using a combination of k*p and tight-binding models. These calculations predict an indirect to direct crossover with the In content and the size of the QDs. The optical properties are then studied in a low-In-content range through photoluminescence and time-resolved photoluminescence experiments. It suggests the proximity of two optical transitions of indirect and direct types
Structural and optical properties of (In,Ga)As/GaP quantum dots and (GaAsPN/GaPN) diluted-nitride nanolayers coherently grown onto GaP and Si substrates for photonics and photovoltaics applications
San Francisco, California, United StatesInternational audienceLattice-matched GaP-based nanostructures grown on silicon substrates is a highly rewarded route for coherent integration of photonics and high-efficiency photovoltaic devices onto silicon substrates. We report on the structural and optical properties of selected MBE-grown nanostructures on both GaP substrates and GaP/Si pseudo-substrates. As a first stumbling block, the GaP/Si interface growth has been optimised thanks to a complementary set of thorough structural analyses. Photoluminescence and time-resolved photoluminescence studies of self-assembled (In,Ga)As quantum dots grown on GaP substrate demonstrate a proximity of two different types of optical transitions interpreted as a competition between conduction band states in X and Î valleys. Structural properties and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown on GaP substrates and GaP/Si pseudo substrates are reported. Our results are found to be suitable for light emission applications in the datacom segment. Then, possible routes are drawn for larger wavelengths applications, in order to address the chip-to-chip and within-a-chip optical interconnects and the optical telecom segments. Finally, results on GaAsPN/GaP heterostructures and diodes, suitable for PV applications are reporte
Comparaison des procédés d'ablation par faisceau laser et par faisceau d'électrons pour la croissance de couches minces
Pulsed techniques for thin film growth show some interesting characteristics over conventional deposition methods, in particular the presence of high energy species involved in the deposition process. Pulsed laser deposition (PLD) is the most well-known and widely used pulsed method and is able to grow thin films of very complex materials, especially oxides. But the technique is limited in the case of wide band gap semiconductors since the laser photon energy might be too weak against the gap of the material leading to thin films with a poor surface morphology. Pulsed electron beam deposition (PED) is a little known growth technique very similar to PLD. The aim of this work is to address and control PED parameters to grow thin films of interest for applications in microelectronics. Each step of the deposition process has been studied and compared to PLD. Electron-matter interaction has been modeled to get the target temperature evolution during the electronic bombardment. The ablation plume has been investigated by optical emission spectroscopy measurments and fast imaging to get species velocities. Zinc oxide (ZnO) has been chosen for this study and thin films of ZnO have been successfully grown by PED. This work show that the film composition is stoichiometric and that their cristalline quality is as good as films grown by nanosecond PLD. The transparency of ZnO films exeeds 80% in the visible range and their conductivity is high enough to consider applications as transparent conducting oxides for example.Les mĂ©thodes de croissance de couches minces dites « pulsĂ©es » prĂ©sentent certaines spĂ©cificitĂ©s, notamment la prĂ©sence d'espĂšces trĂšs Ă©nergĂ©tiques venant se dĂ©poser Ă la surface du substrat. L'ablation laser (PLD) est Ă ce jour la technique pulsĂ©e la plus connue et permet de former des couches minces de composĂ©s complexes, et d'oxydes particuliĂšrement. Mais la mĂ©thode atteint ses limites dans le cas de cibles de matĂ©riaux peu absorbants Ă la longueur d'onde laser comme les semi-conducteurs Ă large bande interdite. L'ablation par faisceau pulsĂ© d'Ă©lectrons (PED) est une technique de croissance encore peu connue et trĂšs similaire Ă la PLD. L'objectif de ce travail est de maĂźtriser les paramĂštres de la PED pour obtenir des couches minces prĂ©sentant un intĂ©rĂȘt pour des applications en microĂ©lectronique. Chaque Ă©tape du processus de dĂ©pĂŽt a fait l'objet d'une Ă©tude et d'une comparaison avec la PLD. La modĂ©lisation de l'interaction Ă©lectron-matiĂšre a permis d'obtenir l'Ă©volution de la tempĂ©rature de la cible soumise au bombardement par les Ă©lectrons. Le plasma d'ablation a Ă©tĂ© Ă©tudiĂ© en dĂ©tail grĂące Ă la spectroscopie d'Ă©mission optique et Ă l'imagerie rapide afin notamment de connaĂźtre l'Ă©nergie des espĂšces du plasma. Le matĂ©riau choisi pour ce travail est l'oxyde de zinc (ZnO). Des films minces de ZnO ont Ă©tĂ© formĂ©s par PED et l'Ă©tude montre que ces couches sont de bonne composition chimique et que la qualitĂ© cristalline des couches est Ă©quivalente aux films formĂ©s par PLD nanoseconde. Ces couches sont transparentes Ă plus de 80% dans le visible et assez conductrices pour imaginer des applications en tant qu'oxyde transparent conducteurs par exemple
Comparaison des procédés d'ablation par faisceau laser et par faisceau d'électrons pour la croissance de couches minces
Pulsed techniques for thin film growth show some interesting characteristics over conventional deposition methods, in particular the presence of high energy species involved in the deposition process. Pulsed laser deposition (PLD) is the most well-known and widely used pulsed method and is able to grow thin films of very complex materials, especially oxides. But the technique is limited in the case of wide band gap semiconductors since the laser photon energy might be too weak against the gap of the material leading to thin films with a poor surface morphology. Pulsed electron beam deposition (PED) is a little known growth technique very similar to PLD. The aim of this work is to address and control PED parameters to grow thin films of interest for applications in microelectronics. Each step of the deposition process has been studied and compared to PLD. Electron-matter interaction has been modeled to get the target temperature evolution during the electronic bombardment. The ablation plume has been investigated by optical emission spectroscopy measurments and fast imaging to get species velocities. Zinc oxide (ZnO) has been chosen for this study and thin films of ZnO have been successfully grown by PED. This work show that the film composition is stoichiometric and that their cristalline quality is as good as films grown by nanosecond PLD. The transparency of ZnO films exeeds 80% in the visible range and their conductivity is high enough to consider applications as transparent conducting oxides for example.Les mĂ©thodes de croissance de couches minces dites « pulsĂ©es » prĂ©sentent certaines spĂ©cificitĂ©s, notamment la prĂ©sence d'espĂšces trĂšs Ă©nergĂ©tiques venant se dĂ©poser Ă la surface du substrat. L'ablation laser (PLD) est Ă ce jour la technique pulsĂ©e la plus connue et permet de former des couches minces de composĂ©s complexes, et d'oxydes particuliĂšrement. Mais la mĂ©thode atteint ses limites dans le cas de cibles de matĂ©riaux peu absorbants Ă la longueur d'onde laser comme les semi-conducteurs Ă large bande interdite. L'ablation par faisceau pulsĂ© d'Ă©lectrons (PED) est une technique de croissance encore peu connue et trĂšs similaire Ă la PLD. L'objectif de ce travail est de maĂźtriser les paramĂštres de la PED pour obtenir des couches minces prĂ©sentant un intĂ©rĂȘt pour des applications en microĂ©lectronique. Chaque Ă©tape du processus de dĂ©pĂŽt a fait l'objet d'une Ă©tude et d'une comparaison avec la PLD. La modĂ©lisation de l'interaction Ă©lectron-matiĂšre a permis d'obtenir l'Ă©volution de la tempĂ©rature de la cible soumise au bombardement par les Ă©lectrons. Le plasma d'ablation a Ă©tĂ© Ă©tudiĂ© en dĂ©tail grĂące Ă la spectroscopie d'Ă©mission optique et Ă l'imagerie rapide afin notamment de connaĂźtre l'Ă©nergie des espĂšces du plasma. Le matĂ©riau choisi pour ce travail est l'oxyde de zinc (ZnO). Des films minces de ZnO ont Ă©tĂ© formĂ©s par PED et l'Ă©tude montre que ces couches sont de bonne composition chimique et que la qualitĂ© cristalline des couches est Ă©quivalente aux films formĂ©s par PLD nanoseconde. Ces couches sont transparentes Ă plus de 80% dans le visible et assez conductrices pour imaginer des applications en tant qu'oxyde transparent conducteurs par exemple
Evolution de la prise en charge de l'infarctus du myocarde chez le sujet de plus de 75 ans de 1983 à 1999 dans un hÎpital général
TOULOUSE3-BU Santé-Centrale (315552105) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF
Root system growth and nodule establishment on pea (Pisum sativum L.)
International audienc
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