J. Perrière

Comparison between ZnO films grown by femtosecond and nanosecond laser ablation

We have studied the structural properties of ZnO thin films grown on Al2O3 (00.1) single-crystal substrates by pulsed-laser deposition using either a femtosecond or a nanosecond laser. Although hexagonal ZnO films deposited on sapphire substrate were epitaxially grown in both cases, the crystalline quality was found to be very different: ZnO films grown with the femtosecond laser are characterized by a higher mosaicity, a smaller crystallite size, a larger content of defects but also smaller residual stresses than ZnO films obtained by nanosecond laser ablation. These differences can be explained according to the kinetic energy of the species evolved during laser ablation as deduced from plasma characterization with a charged-coupled device camera: close to 1 KeV in the femtosecond regime for the population species emitted from the target with the highest velocity, versus a few hundreds of eV in the case of nanosecond pulses. The high energy species irradiation associated with a femtosecond laser is likel...

Er-doped ZnO thin films grown by pulsed-laser deposition

Crystalline erbium(Er)-doped zinc oxide thin films have been grown by pulsed-laser deposition and were analyzed by the complementary use of Rutherford backscattering spectroscopy, x-ray diffraction analysis, atomic force microscopy, and photoluminescence. The composition, structure, and surface morphology of films were studied, as a function of the growth conditions (temperature from 300u2009°C to 750u2009°C and oxygen pressure from 10−6 to 0.5 mbar) and Er-doping rate, and were correlated to the emission spectroscopy of Er in the infrared domain. While these studies lead to the determination of optimal conditions for the growth of high crystalline quality films, results of photoluminescence experiments show that the insertion of Er ions in the ZnO matrix does not follow a simple pattern. The Er ions are incorporated from two pathways, one population is found inside the crystallites and another one at the grain boundaries, as a consequence of the differences in valence and ionic radius of Zn and Er.

Growth of heteroepitaxial ZnO thin films by femtosecond pulsed-laser deposition

ZnO thin films have been grown on various substrates by femtosecond pulsed-laser deposition. According to optical microscopy and atomic force microscopy analyses, the production of droplets is not significant using femtosecond pulses. Smooth, dense, stoichiometric, crystalline, and textured hexagonal ZnO films are epitaxially grown on (0001) sapphire at 700u2009°C with an in-plane epitaxial relationship corresponding to a 30° rotation of the ZnO basal plane with respect to the sapphire. Nevertheless, channeling experiments and rocking curve measurements show that the crystalline quality is not as good as that obtained with nanosecond pulses.

Growth of apatite films by laser ablation: Reduction of the droplet areal density

The pulsed laser deposition of calcium phosphate (apatite) thin films using targets with different optical absorptivities has been studied. It was found that the surface morphology of the grown films greatly depends on the target optical properties, the higher the optical absorption coefficient a, the lower the droplet density on the surface of the deposited films. The temperature profiles inside the different targets during the action of a laser pulse were obtained by numerically solving the heat diffusion equation. These simulations indicated for the low a value targets a large volume heating with the formation of a thick layer of melted material, and the occurrence of a sub‐surface superheating effect once the evaporation began. For higher a value targets, the temperature profiles corresponded to the surface heating case, where the evaporation process began at earlier times and was restricted to the outermost surface region. Time‐ and space‐resolved optical emission spectroscopy investigations showed t...

Formation of GaAs nanocrystals by laser ablation

The pulsed laser ablation method has been used to form GaAs nanocrystals. A quadrupled frequency Nd:yttrium–aluminum–garnet laser beam is focused onto a GaAs single crystal target, and a nitrogen flowing gas is sent at the neighborhood of the target in order to transfer in an ethanol bath, the nanoparticles grown in gas phase. The composition of the particles is close to stoichiometry and transmission electron microscopy analyses highlight zinc-blende GaAs nanocrystals with a rather well defined size: 5–8 nm diameter. The low temperature photoluminescence and photoluminescence excitation spectra show quantum confinement of about 870 meV via an emission band in the visible range (500–560 nm) for the GaAs nanocrystals produced by laser ablation without any postannealing treatment.

LiMn2O4 thin films for lithium ion sensors

Abstract Thin films of LiMn 2 O 4 with the spinel structure have been deposited by the laser ablation method onto various substrates. The composition of the films has been obtained through Rutherford Backscattering methods and Nuclear Reaction Analysis. The Li/Mn ratio strongly decreases with the temperature of the substrate and increases with the oxygen pressure. Pure LiMn 2 O 4 films are obtained under 0.2 mbar at 500°C. For lower oxygen pressures, mixed LiMn 2 O 4 and Mn 3 O 4 crystalline phases are obtained. The films are polycrystalline when deposited onto Si and Pt whereas they are strongly textured when deposited onto MgO. They exhibit high reversible intercalation properties with very good cycling behaviour as shown by cyclic voltammetry. We demonstrate the potential application of such films as very rapid and selective sensors of the Li concentration in aqueous solutions.

Structural and optical characterization of pure Si-rich nitride thin films

The specific dependence of the Si content on the structural and optical properties of O- and H-free Si-rich nitride (SiNx>1.33) thin films deposited by magnetron sputtering is investigated. A semiempirical relation between the composition and the refractive index was found. In the absence of Si-H, N-H, and Si-O vibration modes in the FTIR spectra, the transverse and longitudinal optical (TO-LO) Si-N stretching pair modes could be unambiguously identified using the Berreman effect. With increasing Si content, the LO and the TO bands shifted to lower wavenumbers, and the LO band intensity dropped suggesting that the films became more disordered. Besides, the LO and the TO bands shifted to higher wavenumbers with increasing annealing temperature which may result from the phase separation between Si nanoparticles (Si-np) and the host medium. Indeed, XRD and Raman measurements showed that crystalline Si-np formed upon 1100°C annealing but only for SiNx<0.8. Besides, quantum confinement effects on the Raman peaks of crystalline Si-np, which were observed by HRTEM, were evidenced for Si-np average sizes between 3 and 6 nm. A contrario, visible photoluminescence (PL) was only observed for SiNx>0.9, demonstrating that this PL is not originating from confined states in crystalline Si-np. As an additional proof, the PL was quenched while crystalline Si-np could be formed by laser annealing. Besides, the PL cannot be explained neither by defect states in the bandgap nor by tail to tail recombination. The PL properties of SiNx>0.9 could be then due to a size effect of Si-np but having an amorphous phase.

Growth by laser ablation of Y2O3 and Tm∶Y2O3 thin films for optical applications

Crystalline undoped and thulium doped yttrium oxide (Y2O3) thin films have been grown by the pulsed laser deposition (PLD) technique on various single-crystal substrates heated at 700u2006°C. X-Ray diffraction (XRD) analysis showed that the orientations of the films depended on the substrate and on the oxygen pressure. The crystalline quality of Y2O3 thin films, studied by rocking-curve measurements and by Rutherford backscattering spectroscopy (RBS) in channeling geometry, and the in-plane orientations between films and substrates, determined by X-ray asymmetric diffraction, were also found to depend on the same parameters. The recording of the visible fluorescence spectra of the thulium (Tm) doped Y2O3 thin films, as well as the measurements of the fluorescence lifetimes of the 1D2 and 1G4 thulium emitting levels, gave results very similar to those obtained on bulk crystals. Doping rates in Tm of 0.5 and 1% were found to limit Tm–Tm interactions inside the films and to give relatively high visible fluorescence intensities.

Influence of laser energy density on the plasma expansion dynamics and film stoichiometry during laser ablation of BiSrCaCuO

The plasma expansion dynamics and the composition of films deposited during laser ablation of BiSrCaCuO at laser energy densities in a broad interval (1–6 J/cm2) have been studied by means of spatially resolved real time optical emission spectroscopy and Rutherford backscattering and nuclear reactive analysis, respectively. In vacuum, the velocity of the ejected species is found to increase as the energy density increases whereas the compositional and angular distribution of the deposited films remain unchanged. When an oxygen pressure of 10−1 mbar is applied, the film composition and angular distribution depend on the laser energy density. The results are analyzed in the frame of a collisional mechanism between the ejected and gas species, the effect of increasing the laser energy density being similar to that of decreasing the oxygen pressure. Finally, this competitive influence of the oxygen pressure and the energy density is discussed within the adiabatic expansion model.

Epitaxial growth of Pt and oxide multilayers on MgO by laser ablation

We have studied the epitaxial growth of Pt films on MgO substrates by laser ablation. Depending upon oxygen pressure during deposition, different Pt film textures were observed. Pure (001) films are grown under oxygen (0.1 mbar), while (111) films are formed under high vacuum (<10−6u200ambar). By the complementary use of Rutherford backscattering spectrometry in channeling geometry and x-ray diffraction, the crystalline quality was found nearly perfect. Finally multilayered structures with LiCoO2 or LiMn2O4 layer on Pt film on MgO substrates were epitaxially grown by laser ablation. Such oxide films could present interesting properties as insertion compounds in lithium batteries, and we demonstrate that LiMn2O4 could be used for the accurate determination of lithium concentration in solution.