B. Gallas

SiO2–TiO2 interfaces studied by ellipsometry and x-ray photoemission spectroscopy

We present an in situ study of the formation of the interfaces between TiO2 and SiO2 evaporated thin films using spectroscopic ellipsometry and x-ray photoemission spectroscopy (XPS). The growth of TiO2 on SiO2 was studied previously, but the reverse case has not received much attention up to now. In this article, we show that a common description is valid for both interfaces, which are formed by crosslinking Ti–O–Si bonds. We show also that the growth of TiO2 on SiO2 begins with an amorphous interface layer even when growth occurs at 400 °C. The interface is sharp, a few angstroms, as determined by angular XPS; when SiO2 grows on TiO2, the interface is thicker, about 10 A. Roughness and interdiffusion play roles in interface formation and their role will be discussed.

Metallic clusters in nonstoichiometric gallium oxide films

Oxygen deficient gallium oxide films have been grown by pulsed-laser deposition. Depending on the growth conditions (oxygen pressure and laser power density), amorphous and insulating or partially crystalline and conducting gallium oxide films can be obtained. For very large oxygen deficiencies (Ga2O2.3), the oxide is not stable, and a phase separation leads to metallic Ga nanoparticles embedded in a stoichiometric Ga2O3 matrix. The resistivity measurements evidence the melting and freezing of Ga metallic nanoparticles in addition to their superconducting transition at low temperature. Due to small size both transitions are shifted as compared to the bulk properties.

Negative permittivity and permeability of gold square nanospirals

The optical properties of square nanospirals made of gold have been investigated numerically in the hundred terahertz range as a function of the geometrical parameters of the nanospirals. It is shown that the permittivity values of the nanospirals originate from the longitudinal surface plasmon resonances of the arms composing the nanospirals. A magnetic resonance was also observed whose magnitude and position increased with the number of winding. Above three windings the gold nanospirals exhibited negative values of the real part of the permeability above 400 THz.

Highly dispersive thermo-optical properties of gold nanoparticles

The experimental study of the effect of temperature on the complex optical response of matrix-embedded noble metal nanoparticles has been carried out. A thin silica film containing gold nanoparticles with 7% volume fraction has been elaborated. Its thermo-optical refraction and absorption coefficients have then been extracted from temperature-dependent spectroscopic ellipsometry measurements in the visible range. The results, in agreement with our theoretical approach, exhibit strong dispersion, with amplitude and sign changes due to the local electromagnetic field enhancement associated with the surface plasmon resonance in gold nanoparticles.

Silver square nanospirals mimic optical properties of U-shaped metamaterials

We present a study of the optical properties of three-armed square nanospirals made of silver and realized as nanostructured thin films with Glancing Angle Deposition. Calculation of current flows in the nanospirals show excited resonant modes resembling those observed in U-shaped resonators. Four principal resonances were determined: near 200 THz and 480 THz for one polarization and 250 THz and 650 THz for the polarization orthogonal to the first one. In particular, a mode with anti-parallel current flow in opposite arms, associated with the observed resonance near 650 THz, indicates the existence of a magnetic-like resonance in the square nanospiral arrays. The robustness of the resonances against variations in the structural parameters of the nanospirals was investigated. This study revealed that the main parameter driving the position of the resonances was the overall dimension of the nanospiral, directly related to the length of their arms. Optical properties of a sample were measured by generalized spectroscopic ellipsometry at near-normal incidence, and evidence conversion between polarization states even for light polarized in the plane containing one of the arms in agreement with the numerical study. The measurements compared favorably to the results of the numerical simulations taking into account the disorder in the sample.

Spectroscopic ellipsometry study of a self-organized Ge dot layer

We report on the determination of the dielectric function of a Ge dot layer obtained by epitaxial growth on Si(001) in the presence of Sb as surfactant and capped by Si. After growth, spectroscopic ellipsometry reveals a SiGe alloy containing Ge rich regions. After etching the Si cap, the dielectric function of the Ge rich regions exhibits critical points located at 2.92, 3.65, and 4.25 eV. It is shown that this dielectric function does not correspond either to an alloy or to bulk Ge, and is affected by confinement.

Spray deposition of nanocrystalline Ni1−xZnxFe2O4 (x⩽0.6) films from polyol-mediated sol: Microstructure and magnetic properties

Static-spray deposition of a polyol-based sol, containing nanoparticles of NiZn ferrite, on a moderately heated glass support, has been used to generate crystalline thin films. The films exhibit superparamagnetic behavior with a blocking temperature, which decreases as the zinc content increases, due to their nanocrystalline character. At low temperature, the films are ferrimagnetic; the saturation magnetization and the coercivity depend markedly on both the zinc content and the microstructure.

Ellipsometric investigation of the Si/SiO2 interface formation for application to highly reflective dielectric mirrors

We present a study of the Si/SiO 2 interface formation during the growth of Si on SiO 2 using in situ ellipsometry measurements performed at one wavelength (X = 0.4 μm). Variable angle X-ray photoelectron spectroscopy (XPS) is also used to confirm the ellipsometric measurements. In situ ellipsometry and XPS provide evidence for the formation of a SiO x (0 <x < 2) interlayer between the SiO 2 and Si layers. The variation of x with depth is analysed from (cos Δ, tan ψ) trajectories and the results obtained allow reproduction of the spectroscopic ellipsometry spectra too. The effects of the substrate temperature on the growth are investigated and show that the higher the substrate temperature the larger the interface. The utilisation of a SiO 2 /Si multilayer stack for elaborating an omnidirectional reflector, angle-insensitive in the near infrared range, is also presented.