Eric Millon

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...

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 800u2009°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 700u2009°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...

Phase separation in nanocomposite indium tin oxide thin films grown at room temperature: on the role of oxygen deficiency

A phase separation is found to occur at room temperature in oxygen deficient indium tin oxide films. This disproportionation reaction is driven by the crystallization of the stoichiometric (In1.8Sn0.2)O3 phase as indium tin sub-oxides are not thermodynamically stable. The electrons supplied by the oxygen vacancies may act as reduction centres to induce the formation of metallic In and/or Sn atoms, and when the phase separation is complete, a nanocomposite film is formed with metallic clusters embedded in a stoichiometric oxide matrix. The precise study of the role of oxygen deficiency shows the existence of a threshold in oxygen composition for the occurrence of the phase separation. Indeed for nearly stoichiometric (In1.8Sn0.2)Ox films (with x > 2.75) metallic clusters are not present, while for x < 2.75, metallic clusters are observed. The existence of this threshold is here explained in relation to classical nucleation theory. An analogy is proposed between the critical size leading to the nucleation of metallic clusters and the threshold in oxygen composition.

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 700u2009°C. Pure anatase films are obtained at high oxygen pressure (10−1u2002mbar), while the rutile phase is evidenced at low oxygen pressure (10−5u2002mbar) 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 (>103u2002Ωu2009cm), 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<200u2002K) the resistivity of rutile films versus temperature may be explained by a var...

Electrical and thermal characterization of carbon nanotube films

The remarkable electrical and thermal properties of carbon nanotubes (CNTs) make them attractive for microelectronics applications and, in particular, for interconnects. A multilayer device was designed in order to measure electrical and thermal properties of CNT films. This device is composed of an iron catalyst thin film deposited by pulsed laser ablation upon which a dense multi-walled carbon nanotube (MWCNT) film was grown by radio frequency plasma enhanced chemical vapor deposition. Finally a thin metallic layer was deposited over all by physical vapor deposition. Scanning electron microscopy images were intensively used to check the length (several tens of micrometers) and diameter (10 to 30 nm) of the nanotubes and to adjust the different steps of the process to get the desired film morphology (dense and vertically aligned). The CNT structure was investigated by high-resolution transmission electron microscopy and Raman spectrometry. The MWCNT carpet showed an ohmic behavior during current-voltage ...

Transparent conductive Nd-doped ZnO thin films

Transparent Nd-doped ZnO films with thickness in the range of 70 to 250u2009nm were grown by pulsed-laser deposition (PLD) on c-cut sapphire substrates at various oxygen pressures and substrate temperatures. A wide range of optical and electrical properties of the films were obtained and correlated to the composition and crystalline structure. The Nd-doped ZnO films are smooth, dense, and display the wurtzite phase. Different epitaxial relationships between films and substrate as a function of growth pressure and substrate temperature were evidenced by asymmetric x-ray diffraction measurements. By varying PLD growth conditions, the films can be tuned to have either metallic or semiconductor characteristics, with good optical transmittance in the visible range. Moreover, a low-temperature metal-insulator transition may be observed in Nd-doped ZnO films grown under low oxygen pressure. Resistivities as low as 6 × 10−4u2009Ωu2009cm and 90% optical transmittance in the visible range and different near-infrared transmittance are obtained with approximately 1.0–1.5u2009at.% Nd doping and growth temperature of approximately 500u2009°C.

Thermal conductivity measurement of porous silicon by the pulsed-photothermal method

Thermal properties of two types of porous silicon are studied using the pulsed-photothermal method (PPT). This method is based on a pulsed-laser source in the nanosecond regime. A 1D analytical model is coupled with the PPT technique in order to determine thermal properties of the studied samples (thermal conductivity and volumetric heat capacity).At first, a bulk single crystal silicon sample and a titanium thin film deposited on a single crystal silicon substrate are studied in order to validate the PPT method. Porous silicon samples are elaborated with two different techniques, the sintering technique for macroporous silicon and the electrochemical etching method for mesoporous silicon. Metallic thin films are deposited on these two substrates by magnetron sputtering. Finally, the thermal properties of macroporous (30% of porosity and pores diameter between 100 and 1000 nm) and mesoporous silicon (30% and 15% of porosity and pores diameter between 5 and 10 nm) are determined in this work and it is found that thermal conductivity of macroporous (73 W m−1 K−1) and mesoporous (between 80 and 50 W m−1 K−1) silicon is two times lower than the single crystal silicon (140 W m−1 K−1).

Role of nanostructure on the optical waveguiding properties of epitaxial LiNbO3 films

High-quality LiNbO3 (LN) epitaxial films have been grown on (0 0 1)Al2O3 substrates by pulsed laser deposition. The deposition conditions have been optimized using experimental designs to promote the growth of highly (0 0 1)-oriented, phase pure and light-guiding LN films. The structural and nanostructural properties have been investigated by x-ray diffraction reciprocal space mapping and the guiding properties have been investigated by m-line spectroscopy and measurements of the light propagation losses. In particular it is shown that the film composition, the state of strain, the film thickness, the film roughness, the lateral extension of the crystallites building up the film as well as the mosaicity can be determined by a careful examination of the x-ray reciprocal space maps associated with simulation of the diffracted intensity distribution in reciprocal space. The guiding properties have been correlated with the nanostructural properties of the films: whereas light guiding has been clearly observed in single-crystal-like films, the existence of a mosaic structure, made of nanocrystalline domains, is shown to be detrimental to the guiding properties.

Dynamics of a pulsed-electron beam induced plasma: application to the growth of zinc oxide thin films

Spatial and temporal distributions of species emitted from ablation of zinc oxide target by a pulsed-electron beam are studied by the complementary use of optical emission spectroscopy and camera fast imaging. We show that the slowest electrons produced by the accelerating tube in the pulsed-electron beam deposition (PED) system interact with the plasma plume and involve a further excitation of the neutrals species. The velocities of atoms and ions are found to be similar to those obtained in the well-known pulsed-laser deposition (PLD) process while the plasma temperature and the electron density are lower. The low absorption of the electron beam by the expanding plasma compared with that experienced in PLD leads to a higher removal of ablated material and consequently to a higher deposition rate ten times greater than in PLD. The PED ZnO films grown at room temperature on silicon substrate are constituted by the stacking of small aggregates without large macroscopic droplets. These results are in agreement with the ICCD images recorded at long time delays from the electron impact which do not evidence bright trails correlated with the propagation of slow and large particles.

Tailored electric and optical properties of Nd-doped ZnO: from transparent conducting oxide to photon down-shifting thin films

Nd-doped ZnO films with highly tunable properties were grown by pulsed electron beam deposition at 500 °C on Si and c-cut single crystal substrates under oxygen gas. The effects of a slight change in the oxygen pressure (10−2 to 2 × 10−2 mbar) on the composition, structure and physical properties of the films were studied. Films grown at 10−2 mbar present a low resistivity (5 × 10−3 Ω cm) and high transparency in visible range and do not show any near-infrared emission due to Nd3+ ions. On the contrary, films grown at 2 × 10−2 mbar have high resistivity (>16 Ω cm), high optical transparency and near infrared emission of the Nd3+ ions is observed under indirect excitation at 335 nm (i.e. absorption by the ZnO matrix and transfer to Nd3+ ions). These significant changes in physical properties, leading from transparent conducting oxide to photon down-shifting thin films, are related to growth mode in pulsed-electron beam deposition.