Absence of boron aggregates in superconducting silicon confirmed by atom probe tomography

Superconducting boron-doped silicon films prepared by gas immersion laser doping (GILD) technique are analyzed by atom probe tomography. The resulting three-dimensional chemical composition reveals that boron atoms are incorporated into crystalline silicon in the atomic percent concentration range, well above their solubility limit, without creating clusters or precipitates at the atomic scale. The boron spatial distribution is found to be compatible with local density of states measurements performed by scanning tunneling spectroscopy. These results, combined with the observations of very low impurity level and of a sharp two-dimensional interface between doped and undoped regions show, that the Si:B material obtained by GILD is a well-defined random substitutional alloy endowed with promising superconducting properties.Comment: 4 page

Subkelvin tunneling spectroscopy showing Bardeen-Cooper-Schrieffer superconductivity in heavily boron-doped silicon epilayers

Scanning tunneling spectroscopies in the subKelvin temperature range were performed on superconducting Silicon epilayers doped with Boron in the atomic percent range. The resulting local differential conductance behaved as expected for a homogeneous superconductor, with an energy gap dispersion below +/- 10%. The spectral shape, the amplitude and temperature dependence of the superconductivity gap follow the BCS model, bringing further support to the hypothesis of a hole pairing mechanism mediated by phonons in the weak coupling limit.Comment: 4 pages, 3 figure

Low temperature transition to a superconducting phase in boron-doped silicon films grown on (001)-oriented silicon wafers

We report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by Gas Immersion Laser Doping. The doping concentration cB has been varied up to approx. 10 at.% by increasing the number of laser shots to 500. No superconductivity could be observed down to 40mK for doping level below 2.5 at.%. The critical temperature Tc then increased steeply to reach 0.6K for cB = 8 at%. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field Hc2(0) was on the order of 1000 G, much smaller than the value previously reported by Bustarret et al. in Nature (London) 444, 465 (2006).Comment: 4 pages including 4 figures, submitted to PRB-Rapid Communicatio

UV to NIR photon conversion in Nd-doped rutile and anatase titanium dioxide films for silicon solar cell application

Undoped and Nd-doped titanium dioxide anatase and rutile films have been grown by pulsed-laser deposition at 700 °C under 0.1 mbar O2. By selecting adequate substrates, TiO2 films doped with 1, 2 or 5 at.% Nd were grown and constituted with polycrystalline rutile, highly oriented (2 0 0) rutile film, or oriented (0 0 4) anatase. An UV to NIR photon conversion is evidenced in the films. Indeed, intense and well-resolved emission lines from Nd3+ have been observed upon excitation above the TiO2 bandgap at room temperature. The sensitised emission of Nd3+ is found to be much efficient in rutile than in anatase structure. Low temperature photoluminescence measurements lead to fine resolved peaks corresponding to the Nd3+ 4f transitions with different spectral characteristic according to the host matrix used. Photoluminescence dependence temperature evidences that the light emission from Nd3+ in anatase-based films is probably influenced by the presence of self-trapped excitons or by orbital interaction. Mechanisms of sensitisation host to Nd3+ are proposed for both matrixes. Finally, the Nd dopant concentration and the microstructure of TiO2 rutile films are found to affect the photoluminescence emission intensity. Rutile film (2 0 0) oriented is the most adapted host matrix to sensitise 1 at.% Nd3+ ions for an emission around 1064 nm making such Nd-doped layers interesting for photon conversion by down shifting process

Collisional and thermal ionization of sodium Rydberg atoms I. Experiment for nS and nD atoms with n=8-20

Collisional and thermal ionization of sodium nS and nD Rydberg atoms with n=8-20 has been studied. The experiments were performed using a two-step pulsed laser excitation in an effusive atomic beam at atom density of about 2 10^{10} cm^{-3}. Molecular and atomic ions from associative, Penning, and thermal ionization processes were detected. It has been found that the atomic ions were created mainly due to photoionization of Rydberg atoms by photons of blackbody radiation at the ambient temperature of 300K. Blackbody ionization rates and effective lifetimes of Rydberg states of interest were determined. The molecular ions were found to be from associative ionization in Na(nL)+Na(3S) collisions. Rate constants of associative ionization have been measured using an original method based on relative measurements of Na_{2}^{+} and Na^{+} ion signals.Comment: 23 pages, 10 figure

UV to NIR photon conversion in Nd-doped rutile and anatase titanium dioxide films for silicon solar cell application

Undoped and Nd-doped titanium dioxide anatase and rutile films have been grown by pulsed-laser deposition at 700 °C under 0.1 mbar O2. By selecting adequate substrates, TiO2 films doped with 1, 2 or 5 at.% Nd were grown and constituted with polycrystalline rutile, highly oriented (2 0 0) rutile film, or oriented (0 0 4) anatase. An UV to NIR photon conversion is evidenced in the films. Indeed, intense and well-resolved emission lines from Nd3+ have been observed upon excitation above the TiO2 bandgap at room temperature. The sensitised emission of Nd3+ is found to be much efficient in rutile than in anatase structure. Low temperature photoluminescence measurements lead to fine resolved peaks corresponding to the Nd3+ 4f transitions with different spectral characteristic according to the host matrix used. Photoluminescence dependence temperature evidences that the light emission from Nd3+ in anatase-based films is probably influenced by the presence of self-trapped excitons or by orbital interaction. Mechanisms of sensitisation host to Nd3+ are proposed for both matrixes. Finally, the Nd dopant concentration and the microstructure of TiO2 rutile films are found to affect the photoluminescence emission intensity. Rutile film (2 0 0) oriented is the most adapted host matrix to sensitise 1 at.% Nd3+ ions for an emission around 1064 nm making such Nd-doped layers interesting for photon conversion by down shifting process

Transitions between Rydberg levels of helium induced by electron and neutral collisions

An experimental study of electron- and neutral-induced collisional transfer between Rydberg levels of helium has been made for intermediate values (8 to 17) of the principal quantum number. The techniques of time-resolved, laser induced fluorescence spectroscopy were used to follow the collisional depopulation of the laser pumped level as well as transfers between levels of different principal quantum number in a high purity, well diagnosed helium afterglow. Transfers between singlet and triplet subsystems of atomic helium were found to be negligible. Electron-induced collisional transfer follows closely the functional variation of the Mansbach and Keck hydrogenic theory, but experimental values are on the average smaller by a factor of 0.64 than predicted. Transfers induced by collisions with 3He and 4He atoms are in marked disagreement with Flannery's classical binary-encounter theory for the smaller values of the principal quantum number, although disagreement is less pronounced for higher principal quantum numbers. This fact, and the absence of isotopic effect on rate coefficients, indicate that a simple binary-encounter theory is not suitable for intermediate Rydberg levels, and that interatomic potentials should be included in a two-centre, molecular model with an excited electron.Nous donnons les résultats d'une étude expérimentale des transferts induits par collisions atomiques et électroniques entre niveaux de Rydberg de l'hélium ayant des nombres quantiques principaux compris entre 8 et 17. La technique de fluorescence induite par laser a été utilisée pour suivre avec une résolution temporelle suffisante le dépeuplement collisionnel du niveau pompé ainsi que les transferts entre niveaux de nombres quantiques différents dans une post-décharge d'hélium de haute pureté dont les différents paramètres sont mesurés avec précision. Nos observations montrent que les transferts entre sous-systèmes triplet et singulet sont négligeables. Les transferts dus aux collisions avec les électrons suivent étroitement l'allure fonctionnelle prévue par la théorie hydrogénique classique de Mansbach et Keck, mais les valeurs expérimentales sont en moyenne plus petites d'un facteur 0,64. Les transferts induits par les collisions avec des atomes de 3 He et de 4He sont en net désaccord avec la théorie classique de Flannery, qui utilise le modèle de la collision binaire ; cependant, le désaccord s'atténue pour les nombres quantiques principaux les plus élevés. Ce fait, joint à l'absence d'effet isotopique sur les taux, indique qu'un modèle simple de collision binaire décrit mal les niveaux de Rydberg moyennement excités, et qu'il est nécessaire de tenir compte des potentiels interatomiques dans un modèle moléculaire à deux centres, plus un électron excité

TRANSFERTS D'EXCITATION ATOMIQUE PAR COLLISIONS ELECTRONIQUES

Nous décrivons les résultats des études théoriques et expérimentales d'excitation atomique par collisions électroniques. L'accent est mis sur les transferts mettant en jeu des états très excités. Les applications aux mécanismes de recombinaison et d'ionisation dans les plasmas seront soulignées.Results of theoretical and experimental studies concerning atomic excitation by electron impact are discussed. The main emphasis is on excitation transfer between highly excited (Rydberg) states. The usefulness of these data for the interpretation of ionization and recombination processes in plasmas will be particularly underlined

Direct synthesis of titanium nitride by laser-plasma

No abstract availabl