Showing papers by "Francis Lévy published in 1994"
TL;DR: In this article, the metastable phase anatase has been shown to have a wider optical absorption gap than rutile thin films, which is consistent with the high mobility, bandlike conduction observed in anatase crystals.
Abstract: Electrical and optical spectroscopic studies of TiO2 anatase thin filmsdeposited by sputtering show that the metastable phase anatase differs in electronic properties from the well‐known, stable phase rutile. Resistivity and Hall‐effect measurements reveal an insulator–metal transition in a donor band in anatase thin films with high donor concentrations. Such a transition is not observed in rutile thin films with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self‐trapped excitons is observed in anatase thin films, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin films have a wider optical absorption gap than rutile thin films.
1,560 citations
TL;DR: In this paper, the experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literature for anatase crystals, and shows some characteristics similar to the experimental DOS reported for rutile crystals.
Abstract: Photoemission spectromicroscopy was used to investigate the electronic structure of TiO2 anatase single crystals and polycrystalline thin films The stoichiometry and the degree of oxidation of as-grown crystals, as-deposited films, as well as of thermally annealed samples in different atmospheres, were analyzed, based on the Ti 2p and O 1s core levels, with an energy resolution of 04 eV The experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literature for anatase crystals, and shows some characteristics similar to the experimental DOS reported for rutile crystals In reduced samples, the experimental DOS is characterized by intense emission in the region of O 2p bonding orbitals, and does not exhibit an appreciable density of states in the band gap As-grown crystals exhibit small band gap emission (a few percent of the valence band VB signal) at about 08 eV, which is attributed to Ti3+ (3d) defect states Annealing the crystals at high temperatures in O2 or subsequent thermal reduction in an Ar-H2 Mixture (95%-5%) produces nearly stoichiometric surfaces with smaller or undetectable density of Ti3+ States In addition, some redistribution of the spectral weight is observed in the VB spectra
664 citations
TL;DR: In this article, resistivity, thermopower, and Hall-effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n-type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states.
Abstract: Resistivity, thermopower, and Hall‐effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n‐type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states. The decrease of the mobility with increasing temperature is consistent with the scattering of carriers by the optical phonons of TiO2.
462 citations
TL;DR: In this paper, a nonreactive, reactive, and low energy ion-assisted radio-frequency (RF) magnetron sputtering from a MoS2 target was used to create a lubricating thin film.
Abstract: MoSx lubricating thin films were deposited by nonreactive, reactive, and low energy ion‐assisted radio‐frequency (rf) magnetron sputtering from a MoS2 target. Depending on the total and reactive gas pressures, the film composition ranges between MoS0.7 and MoS2.8. A low working pressure was found to have effects similar to those of low‐energy ion irradiation. Films deposited at high pressure have (002) planes preferentially perpendicular to the substrate, whereas films deposited at low pressure or under low‐energy ion irradiation have (002) mainly parallel to it. Parallel films are sulfur deficient (MoS1.2−1.4). Their growth is explained in terms of an increased reactivity of the basal surfaces, itself a consequence of the creation of surface defects due to ion irradiation. The films exhibit a lubricating character for all compositions above MoS1.2. The longest lifetime in ball‐on‐disk wear test was found for MoS1.5.
71 citations
TL;DR: In this paper, the structure of sputter-deposited MoS2−x films is investigated by X-ray and electron diffraction and the diffraction spectra show important differences with respect to those of known molybdenite phases.
50 citations
TL;DR: Vortex pinning by columnar defects in a NbSe crystal irradiated with 2.2 GeV ions is studied by scanning tunneling microscopy (STM) at 3 K and the whole vortex arrangement is disordered, although less random than the defect distribution.
Abstract: Vortex pinning by columnar defects in a ${\mathrm{NbSe}}_{2}$ crystal irradiated with 22 GeV ${\mathrm{Au}}^{+24}$ ions is studied by scanning tunneling microscopy (STM) at 3 K The vortex arrangement is clearly resolved in magnetic fields up to 500 mT and compared to the defect locations, also extracted from STM data At vortex densities much larger than the defect density, the triangular vortex lattice is perturbed close to the defects At vortex densities comparable or smaller, only some defects trap a vortex and the whole vortex arrangement is disordered, although less random than the defect distribution
45 citations
TL;DR: Amorphous Nb2O5 thin films of three different thicknesses (10, 100, 400 nm) were deposited onto SiO2/Si substrates by reactive sputtering in an Ar-O2 plasma as discussed by the authors.
Abstract: Amorphous Nb2O5 thin films of three different thicknesses (10, 100, 400 nm) were deposited onto SiO2/Si substrates by reactive sputtering in an Ar–O2 plasma. Thermal treatments were performed at different temperatures between 500 and 1100 °C. The structural and morphological evolution with temperature is shown to be dependent on the film thickness. At 600 °C, the films essentially crystallize in the TT phase. On the thickest films, the T phase also appears. Annealing at higher temperature progressively increases the concentration of the T phase. The films show large flat grains extending over the whole film thickness. In addition, a large number of polyhedral bubbles is present in the 100 and 400 nm films due to Ar atoms trapped during sputtering. After annealing at 1100 °C the Ar bubbles are no longer present and partial diffusion of the films into the substrate is observed. The modification at high temperature, explained either by the M or the H phase, is favored on the thickest films and leads to plate...
40 citations
TL;DR: In this article, the structures of MoS 2 thin films grown in various conditions were studied by transmission electron microscopy in combination with complementary investigations (X-ray diffraction, electron probe microanalysis, sliding lifetime, etc.).
Abstract: The structures of MoS 2 thin films grown in various conditions were studied by transmission electron microscopy in combination with complementary investigations (X-ray diffraction, electron probe microanalysis, sliding lifetime, etc.). The morphology and orientation of the films depend on the deposition temperature and gas pressure. They are related to the chemical composition defined by the (reactive) sputtering process parameters. Two types of film deposited at high temperature (100-500°C) are structurally distinguishable as a function of the stoichiometry ratio x = C s /C Mo . Films deposited at low pressure (LP) with 1.2 < × < 1.5 are dense with basal (002) planes oriented parallel with the substrate. For 1.5 < x < 2.8, near the interface, the (002) planes are also oriented parallel with the substrate. However, after 5-10 nm of growth, a 90° change occurs in the crystalline orientation. The films deposited at high pressure (HP) exhibit a porous lamellar morphology with the (002) planes perpendicular to the substrate. Films deposited at low substrate temperature or with a large suIphur deficiency (x < 0.9) are almost or totally amorphous. Ball-on-disc tribometry tests in a dry atmosphere show that the crystallized coatings (x ≥ 1.2) act as lubricants friction coefficient, 0.02-0.05). The sliding lifetime is at a maximum for 1.4 < x < 1.7. Cross-sectional transmission electron microscopy of wear tracks suggests wear mechanisms for both types of high-temperature film. In the lamellar films (HP), the friction produces a buffer layer, with significant densification and reorientation of the crystallites sliding along basal planes. In the compact films (LP), the tribological test leads to a decrease in the thickness of the crystallites in the buffer layer. The tribological properties of understoichiometric molybdenite are better than those of stoichiometric coatings
28 citations
TL;DR: In this article, the optical properties of transition metal nitrides are evaluated by ex-situ ellipsometry using the screened Drude model, and they are correlated with the differences in composition.
Abstract: In the investigation of thin films of transition metal nitrides, an essential role is played by the accurate determination of their chemical composition. Actually the chemical composition depends on the deposition parameters and influences the optical properties. These relations are illustrated in thin films of TiN x and (Ti 1− y V y )N x deposited by reactive magnetron sputtering from composite targets of the elements. By variation of the nitrogen partial pressure and the target composition, different samples have been obtained. The chemical composition has been measured by electron probe microanalysis at low irradiation voltages. The optical properties are evaluated by ex-situ ellipsometry. Using the screened Drude model, they are correlated with the differences in composition. Adding vanadium or nitrogen in Ti-N is shown to have the same effect on the optical properties.
19 citations
TL;DR: In this article, the electronic properties of amorphous and ion-implanted polycrystalline SnO2 films have been studied using x-ray photoemission spectromicroscopy.
Abstract: The electronic properties of amorphous and ion-implanted polycrystalline SnO2 films have been studied using x-ray photoemission spectromicroscopy. First, we analysed the degree of oxidation and the homogeneity of as-deposited films. The SnO and SnO2 phases were identified from their valence band spectra rather than from the Sn 3d core level spectra. Then we studied the damage effects due to the implantation of Pd+ and Ga+ ions in polycrystalline films. The ion bombardment of SnO2 films results in drastic changes of valence-band spectra which is correlated to compositional changes. Annealing in air at 600-degrees-C for 4 leads to complete recovery of radiation damage. However, after such thermal annealings, the film surface still contains a relatively high amount of SnO (5-10%).
14 citations
24 Jul 1994
TL;DR: In this article, single crystals of BaVS 3 were synthesized by a new technique; grown in melted tellurium; the crystals were obtained in the form of large needle with typical dimensions of about 10mm in length and 1mm in width.
Abstract: Single crystals of BaVS 3 were synthesized by a new technique; grown in melted tellurium. The crystals were obtained in the form of large needle with typical dimensions of about 10mm in length and 1mm in width. The electrical resistivity gradually decreased with decreasing temperature from 300K to 130K, which clearly showed a metallic behavior, but steepy increased at 70K. It was firstly found out that a change of the gradient in the temperature dependent resistivity occurred at 250K. This is caused by a structural transformation, demonstrating the crystals grown by our technique with high-quality.
TL;DR: In this article, the photoelectrochemical properties of Re 6 Se 7 Br 4 were investigated in aqueous electrolytes and the spectral response indicated an indirect band gap of about 1.78 eV for the compound.