Yves Mathey

XAS, XRD, AFM and Raman studies of nickel tungstate electrochromic thin films

Abstract Systematic studies of nanocrystalline nickel tungstate, NiWO 4 , thin films were performed by several experimental techniques such as Ni K- and W L 1,3 -edges X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, atomic force microscopy and cyclic voltammetry measurements. We found that the NiWO 4 thin films exhibit electrochromic properties similar to that of amorphous tungsten trioxide films, but show better structural stability upon multiple colouring/bleaching cycling. It was observed that a nanocrystallinity of the thin films results in strong modifications of the NiO and WO interactions, which affect both local atomic and vibrational structures.

Deposition of Y-Ba-Cu oxide superconducting thin films by Nd:YAG laser evaporation

Abstract We report on the deposition of Y-Ba-Cu-O superconducting thin films using infrared Nd:YAG laser irradiation. The observed spatial variation of the film thickness can be approximated by a cos 6 θ law, where θ is the angle from the target surface normal. The time-of-flight measurements show that multiple ionisation-excitation processes of the ejected species occur during laser evaporation.

Pulsed laser deposition of NbTex thin films

Abstract Pulsed laser deposition (PLD) of niobium telluride was performed by vaporisation of NbTe2 bulk targets with subsequent deposition of the ablation plume on heated silicon substrates. The laser energy density onto the target and the temperature of the substrate appear to play a major role in the quality of the deposited films. Correlations between these two parameters and the film composition on the one hand, and the structural properties on the other, were sought by systematic elemental and X-ray diffraction analysis. The deposition conditions for well-crystallized PLD films belonging to the Nb5Te4 (1D) structural type with strong (110) preferential orientation were demonstrated for the first time.

Pulsed laser deposition of thin film materials in vacuum and in ambient gas conditions

Pulsed laser ablation has attracted great attention over the past few years as promising technique for depositing thin films. A large variety of successful experimental results were obtained in this field, including the growth of high- temperature superconducting films, ferroelectric films, oxides, semiconductors, diamonds, etc. One of the main advantages of this technology is the simplicity of the experimental set-up. In a common configuration, the laser- generated flux of particles is collected on a planar substrate positioned parallel to the irradiated surface. Several modifications, like simultaneous generation of two plumes from different targets (double ablation), were proposed. Different lasers (e.g. KrF with (lambda) equals 248 nm, Nd-YAG with (lambda) equals 532 nm, etc.) with energy density 2 - 10 J/cm2 were used in the ablation experiments both in vacuum and into diluted ambient gas (pressure up to 750 mTorr). Monte Carlo simulation was found to be a successful technique for theoretical investigations of the laser ablation processes. This method has allowed us to investigate the influence of elastic collisions and chemical reactions in the laser ablated plume on the angular characteristics of the flow. The results of the simulation show that elastic collisions give rise to focusing of particles towards the surface normal and to the redirection of the velocities of the more energetic particles in the direction close to the surface normal. The chemical reactions are found to influence the angular distributions in the way opposite to the one of elastic collisions. The reaction heat contributes to the energy of particles and the velocity distributions are affected by reactions. As result of these processes, the angular distributions are broadened from the surface normal. Additional collisions with the particles of the ambient gas were shown to influence the composition and uniformity of thin films. The study of these processes is of a particular interest for the developing of pulsed laser deposition (PLD) technique.

Lithium insertion electrodes based on niobium telluride thin films

Niobium telluride thin films were evaluated as intercalation electrodes in lithium cells. The films were prepared on Al substrate by sputter deposition under different experimental conditions. Large deposition periods lead to a non-crystalline film material, while short deposition periods allow the formation of layered particles structurally similar to niobium ditelluride. For preparations carried out at 300°C, the particles have sizes of ca 1 μm with their 001 planes oriented parallel to the substrate. At 330°C, particles of ca 10 μm with their 001 axis parallel to the substrate are obtained. Test batteries of the type Li/LiClO4(PC)/niobium telluride film were studied by potentiostatic and galvanostatic procedures. The performance of the batteries is influenced by the preparation conditions. Cell capacities larger than 100 mA h g−1 and a good capacity retention after 50 cycles were observed for films prepared at 300°C when the cells were cycled between 0.5 and 2.0 V.

Anisotropic electrical and optical behavior and preferential orientation in early transition metal tellurides thin films

Highly anisotropic polytellurides MTe with M Zr and x 1. 5 to 5 have been synthetized under thin film form for the first time. Preferential orientations of these films with respect to the substrate surface are evidenced and tentatively correlated to the anisotropy of the optical or conduction properties.

Niobium and zirconium telluride thin films prepared by sputtering

A versatile procedure of sputter deposition, well-adapted for getting a large range of Te/M ratios (with M = Zr or Nb), has led to the synthesis of several highly anisotropic zirconium and niobium polytellurides in thin film form. Upon tuning the two key parameters of the process, i.e., the Te percentage in the target and the substrate temperature during the deposition, preparation of systems ranging from ZrTe 0.72 to ZrTe 6.7 , on the one hand, and from NbTe 1.28 to NbTe 7.84 , on the other, has been achieved. Besides their amorphous or crystalline (with or without preferential orientations) behavior and their relationship to known structural types, the most striking feature of these films is their large departure from the stoichiometry of the bulk MTe x reference compounds. This peculiarity, together with the possible changes of composition under annealing, are described and interpreted in terms of variable amounts of Te and M atoms trapped or intercalated within the parent structures.

XAFS studies of local atomic structure of niobium tellurides

Abstract We present XAFS (K-edge) studies of the local atomic structure around the Nb and Te ions in niobium tellurides (NbTe 2 , Nb 3 Te 4 and NbTe 4 ) in comparison with ZrTe 2 . Using a multi-shell best-fit analysis procedure, we have reconstructed local environments of the Nb and Te ions and compared them with the existing structural models. We found that at the Nb K-edge the XAFS data of NbTe 4 are extremely sensitive to metal clustering and to the subsequent departure from the average crystallographic positions. Moreover, at the Te K-edge the XAFS data allowed us to distinguished the different type of TeTe bonds, for instance: the dimer (TeTe: 2.9 A) and the squares (TeTe: 3.3 A) in NbTe 4 , the interlayer and intralayer (TeTe: 2.9–4.0 A) in NbTe 2 , through their Debye-Waller characteristics. In addition, a comparative analysis of Debye-Waller factors obtained in the different coordination shells (TeTe, TeNb, NbTe and NbNb) shows an evidence of correlation in the vibrational motion.

Characterization of ablation plume during pulsed laser deposition of NbTex thin films

A large field of applications as microelectronics, micromechanics and optics needs to overcome the deposition of various materials showing dielectric, superconducting, piezoelecinc properties under thin film form. Numerous methods were developed as Chemical Vapour Deposition, Sputtering, Thermal Evaporation. Since the early 1980s, a new process based on laser ablation is developed.1 The Pulsed Laser Deposition (P.L.D) method is based on the laser evaporation of a target and the subsequent deposition of the ablation plume on a substrate (see fig. 1). A wide variety of materials was successfully deposited in thin film form by this method. Recently, Nb5Te4 thin films were realized for the first time by using P.L.D process.3 The Nb5Te4 compound presents strong anisotropic properties and finds applications in microelectronics (1D conductors) and micromechanics (solid lubricant). These first results3 shown the key roles played by the laser fluence and the substrate temperature on the film composition and crystallisation. For example, increasing the laser fluence decreases the interreticular distance observed by X-ray diffraction and increases the Te/Nb ratio. This shows the importance of kinetic parameters of the laser-induced plume on the crystallisation process. In order to understand the influence of the laser parameters on the film formation, it is necessary to study the ablation process and the expansion of the ablation cloud.

Microstructure of YBaCuO superconducting thin films deposited by high power and high repetition rate excimer lasers

Abstract The deposition parameters used for the preparatiion of thin YBaCuO films on MgO substrates by the pulsed laser ablation method are of the utmost importance in controlling the microstructure. The film quality is mainly controlled by the deposition rate, the laser fluence and the substrate temperature. The best quality films are produced for low enough values of the deposition parameters where well-oriented grain growth occurs with the c axis perpendicular to the substrate surface. Higher values of the deposition parameters result in higher nucleation rates leading to a continuous loss of epitaxy up to the formation of randomly oriented nanocrystalline films and hence to a strong current leakage at grain boundaries which lowers the critical temperature.