Wilfrid Seiler

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 300 °C to 750 °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 700 °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.

Metal-semiconductor transition in epitaxial ZnO thin films

We report on the formation and transport properties of ZnO thin films which are grown by pulsed-electron beam deposition under a low residual oxygen pressure (10(-5) mbar). ZnO films presenting metallic conductivity at room temperature, and a metal-semiconductor transition at low temperature, were epitaxially grown on Al(2)O(3) single crystal substrates for growth temperatures in the 300-450 degrees C range. These results have been interpreted through the quantum corrections to conductivity in a disordered oxide conductor, implying first a high density of carriers leading to degenerate semiconductor ZnO films, and then a sufficient disorder in these films. These characteristics could be related to the nature of the ZnO films formed by pulsed-electron deposition: a high density of carriers related to an oxygen deficiency in the films and a high density of defects related to the high deposition rate of the pulsed-electron beam deposition method

Structural and optical properties of rare-earth-doped Y2O3 waveguides grown by pulsed-laser deposition

Crystalline rare-earth-doped yttrium oxide thin films were grown by pulsed-laser deposition (PLD) on SiO2/Si substrates. The structural and morphological features of these films were studied, as a function of the growth conditions (temperature from 200 to 800 °C and oxygen pressure from 10−6 to 0.5 mbar), by using Rutherford backscattering spectroscopy, x-ray diffraction, and atomic force microscopy. The related optical properties were investigated by m-lines spectroscopy at 633 nm and 1.3 μm. The optimal conditions were found to be a temperature and a pressure of 700 °C and 10−6 mbar, respectively. In that case, the Y2O3 films are stoichiometric with controlled erbium and europium rates, and present a well-crystallized, (111) textured cubic phase and a low surface roughness of about 10 A. Moreover, the PLD films show good waveguiding properties with a high refractive index (1.92 at 633 nm), a step-index structure, and low optical losses around 1 dB/cm in the near infrared region, promising for a planar a...

Microstresses in textured polycrystals studied by the multireflection diffraction method and self-consistent model

A new version of the X-ray diffraction method for determining macrostresses and microstresses in textured polycrystalline material is presented. In this method the lattice strains for various orientation of the scattering vector as well as for various crystallographic planes {hkl} are measured. The interpretation of the experimental data is based on the least-squares fitting procedure, in which the diffraction elastic constants and theoretical values of microstresses are used. The diffraction elastic constants and the microstresses are calculated by the self-consistent model. The new method was successfully applied for stress determination in one- and two-phase steels subjected to elastoplastic deformation, and the significant anisotropy of the incompatibility stresses was observed in textured samples.

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 (<10−6 mbar). 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.

Nanocomposite indium tin oxide thin films: formation induced by a large oxygen deficiency and properties

We report on the formation and properties of nanocomposite indium tin oxide thin films which are grown by pulsed-electron beam deposition under a low oxygen pressure leading to the formation of highly non-stoichiometric indium tin oxide films. For growth at room temperature these films are amorphous and insulating, while at higher temperatures, the oxygen deficiency leads to a disproportionation reaction with the formation of metallic clusters (indium or indium tin clusters) embedded in a stoichiometric crystalline indium tin oxide. This matrix is well crystallized and even epitaxial for growth on c-cut sapphire single crystal substrates. The presence of the metallic clusters induces specific transport properties, i.e. a metallic conductivity at room temperature followed by a superconducting transition at low temperature (about 6 K). Moreover, the solid-liquid and liquid-solid phase transitions in the metallic clusters can be clearly seen from the resistivity curves as a function of temperature (in the room temperature to 450 K range), through specific changes in resistivity and the appearance of a hysteresis cycle.

Epitaxial PbTiO3 thin films grown on (100) MgO by pulsed-laser deposition for optical waveguiding properties

Perovskite single phased PbTiO3 thin films have been grown by pulsed-laser deposition on (100) MgO single crystals in order to prepare high quality optical waveguides. The substrate temperature and the oxygen pressure during the deposition have been optimized to improve crystalline quality and to grow epitaxial thin films. The films display two main axis orientations according to the MgO (100) basal plane. The waveguiding properties and the optical losses characterized by using m-lines spectroscopy and charge-coupled device camera measurement, respectively, are correlated to the structural properties of the films. Such textured and epitaxied films display very good optical properties, as the optical losses are found to be lower than 1 dB cm−1.

Structural, optical, and electrical properties of epitaxial titanium oxide thin films on LaAlO3 substrate

Titanium oxide thin films were prepared by pulsed-laser deposition on LaAlO3 single crystal substrate at 700 °C. Pure anatase films are obtained at high oxygen pressure (10−1 mbar), while the rutile phase is evidenced at low oxygen pressure (10−5 mbar) despite a large oxygen deficiency (O/Ti=1.75). From asymmetric x-ray diffraction measurements, the in-plane epitaxial relationships be0tween the substrate and the titanium oxide phases are highlighted. Optical constants (refractive index n and extinction coefficient k) were deduced from ellipsometric measurements. The optical band gap energies of the anatase and rutile films are found to be 3.4 and 3.3 eV, respectively. Since the nearly stoichiometric anatase films are resistive (>103 Ω cm), the large oxygen deficiency in rutile films leads to noticeable increase in the conductivity due to the Ti3+ species, which supply electrons in the conduction band. At low temperature (T<200 K) the resistivity of rutile films versus temperature may be explained by a var...