Publications (30)44.51 Total impact
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Article: Disorder in optical metamaterials made of silver nanospirals
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ABSTRACT: We report on a numerical study of the optical properties of silver square nanospirals. The resonant modes of the nanospirals presented current distributions similar to those of U-shaped resonators. The resonance frequencies of the ‘electric’ modes were relatively insensitive to coupling, except for the shortest distance, whereas for the ‘magnetic’ modes they steadily increased with coupling. Aperiodicity in the strong coupling regime did not modify the strength and resonance frequency of all modes as compared to the case of periodic arrays.Applied Physics A 05/2012; 103(3):783-787. · 1.63 Impact Factor -
Article: Silver square nanospirals mimic optical properties of U-shaped metamaterials.
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ABSTRACT: 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.Optics Express 08/2010; 18(16):16335-44. · 3.59 Impact Factor -
Article: Evolution of the optical properties of Si nanoparticles embedded in SiO2 as function of annealing conditions
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ABSTRACT: The dielectric function of Si nanoparticles embedded in silica has been determined from spectroscopic ellipsometry and photothermal deflexion spectroscopy from 0.7 to 6 eV . The influence of crystalline fraction and diameter of the nanoparticles on their optical properties has been investigated. Above 4 nm of diameter, the nanoparticles presented a dielectric function similar to that of fine grained polycrystalline Si (poly-Si) at photon energy higher than 2 eV , with the well marked structures associated with the E<sub>1</sub> and E<sub>2</sub> critical points. In contrast, below 2 eV their absorption coefficient was smaller than for poly-Si. Below 2.5 nm of diameter, the dielectric function of the nanoparticles drastically changed. The magnitude of the imaginary part of the dielectric function of the nanoparticles near the position of the E<sub>1</sub> critical point constantly decreased, whereas it increased at the position of the E<sub>2</sub> critical point. These observations can be interpreted as the result of the transfer of the oscillator strength of the low energy states to the high energy states as the diameter of the nanoparticles decreases. The states associated with the fundamental indirect gap are slowly blueshifted when the diameter of the nanoparticles decreased, as evidenced by photoluminescence measurements.Journal of Applied Physics 07/2008; · 2.17 Impact Factor -
Conference Proceeding: Study of Erbium Doped Silica Films Containing Silicon Nanograins
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ABSTRACT: SiO<sub>x</sub>(1<x<2) film doped with 0.11 at % Er was prepared by electron gun evaporation. Three successive annealing steps at 850, 900 and 1000degC, 1 hour for each step, were performed to achieve a phase separation between Si and SiO<sub>2</sub>. The 900degC annealing step did induce the phase separation, observed by the ellipsometry measurement. With respect to the 900degC annealing step, the 1000degC annealing step completed the decomposition, red-shifted the peak energy of the Si-related PL band, and decreased the intensity of the Er PL band. The Er effective excitation cross sections were successfully determined, which were around 1*10<sup>-16</sup> cm<sup>2</sup>. The Er cross section decreased with increasing excitation wavelength, in good agreement with the observation of photoluminescence excitation (PLE) spectra. With more annealing steps, the Er cross section slightly increased. In addition, we have employed an oxidation procedure to obtain an Er doped SiO<sub>2</sub> film. After oxidized, the Si-related PL band disappeared and the Er PL intensity was extremely reduced indicating the Er ions were principally excited by the energy transferred via silicon nano grains.Electron Devices and Solid-State Circuits, 2007. EDSSC 2007. IEEE Conference on; 01/2008 -
Article: Interfacial diffusion effect on phase transitions in Al/Mn multilayered thin films
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ABSTRACT: Thin films of Al and Mn multilayers were synthesized using thermal evaporation under high vacuum conditions. The whole film thickness containing three bilayers of Al and Mn is about 120nm. The global concentration of the samples was varied between 10 and 46.5 at.% Mn, by changing the thickness of the bilayer. The as-evaporated samples were heat treated at different temperatures (473, 623, 823 and 873K) for 2 and 8h to investigate the interfacial diffusion induced phase transformations in the multilayered thin films. Transmission electron microscopy (TEM) has been mainly used to characterize the crystalline structure of a variety of phases revealed on annealing, such as μ, λ and φ phases up to 823K, δ phase at 823K and T6 phase at 873K. The occurrence of a variety of structures on annealing has been attributed to the interfacial reactions at the Al–Mn bilayers, and, therefore, the global composition of the composite films is not significant during the process of phase transformations. The crystallographic relationships of Al–Mn approximant structures of the decagonal quasicrystal are discussed to understand the evolution and stability of the T6 phase at high temperature.Journal of Materials Science 12/2006; 42(1):185-190. · 2.02 Impact Factor -
Article: Optical properties of Si nanocrystals embedded in SiO_ {2}
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ABSTRACT: The dielectric function of Si nanocrystals embedded in a SiO2 matrix has been determined in the 1.6–6.2 eV spectral range without any assumption on the dispersion law. The Si nanocrystals have been obtained by thermal annealing of SiOx layers yielding nanocrystals diameters of 4.5 nm and 1 nm for atomic composition values x of 1.1 and 1.9, respectively. The dielectric function of the large Si clusters exhibited three structures located at 3.5 eV, 4.2 eV, and 5.4 eV ascribed to the E1, E2, and E1′ critical points, respectively. For the smaller Si clusters, the structure associated with the E1 CP, near 3.5 eV, disappeared while those at higher energy remained present.Phys. Rev. B. 10/2005; 72(15). -
Article: Correlation between Si-related and erbium photoluminescence bands and determination of erbium effective excitation cross section in SiO2 films
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ABSTRACT: Er-doped SiOx (x ∼ 1) films were prepared by electron gun evaporation. After thermal annealing at 1000 °C, all samples showed a Si-related photoluminescence (PL) band at 920 nm (1.3 eV); the samples containing Er showed an Er PL band at 1530 nm. The correlation between these two PL bands was studied by time decay measurements of the Si-related PL band. The lifetime of the Si-related PL band was reduced by Er doping. This reduction increased with increasing Er concentration. The Er effective excitation cross section in SiO2 films containing Si nanocrystals was estimated to be in the order of 10−16 cm2 and showed a dependence on the Er concentration.Journal of Applied Physics 07/2005; 98(1):013544-013544-5. · 2.17 Impact Factor -
Article: Spectroscopic ellipsometry study of a self-organized Ge dot layer
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ABSTRACT: 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. © 2003 American Institute of Physics.Journal of Applied Physics 08/2003; 94(4):2248-2253. · 2.17 Impact Factor -
Article: SiO2–TiO2 interfaces studied by ellipsometry and x-ray photoemission spectroscopy
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ABSTRACT: 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 Å. Roughness and interdiffusion play roles in interface formation and their role will be discussed. © 2002 American Institute of Physics.Journal of Applied Physics 08/2002; 92(4):1922-1928. · 2.17 Impact Factor -
Article: Making an omnidirectional reflector.
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ABSTRACT: The effect of having a finite number of layers on the design of omnidirectional reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-wave to increase reflectivity, whereas layers below may remain of quarter-wave optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO(2) (amorphous silicon and silicon dioxide) omnidirectional reflectors in the near-infrared range on a silicon and a silica substrate, respectively. Owing to the decrease of absorption of recrystallized silicon as compared with a-Si in the visible range, the transmissivity of the structure realized on silica substrate was dramatically increased in the visible range upon annealing, whereas the high reflectivity and the omnidirectional effect were maintained in the near-infrared range.Applied Optics 11/2001; 40(28):5056-63. · 1.41 Impact Factor -
Article: Change of TO and LO mode frequency of evaporated SiO2 films during aging in air
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ABSTRACT: The microstructural modifications, induced by atmospheric water vapor, in the silica network of porous amorphous SiO2 films, evaporated by electron gun and capped by a nonporous TiO2 layer, have been investigated. We have taken advantage of infrared ellipsometric measurements to extract the infrared complex dielectric function of the SiO2 films in the 600–5000 cm−1 range, from which we deduce the actual values of the frequencies of the Si–O–Si stretching vibrational modes [transverse optic (TO) and longitudinal optic (LO)]. The TO and LO frequency shifts are studied experimentally and reproduced by simulation. Experimental data show the increase as a function of time in air of both TO and LO frequencies, and of the refractive index in the visible range. The film is modeled as a mixture of two constituents: a silica matrix, with variable density, and pores, into which water can penetrate. We show that the TO frequency is mainly sensitive to the silica matrix density, while the LO frequency gives information about the pore volume fraction. We first demonstrate that our deposition method leads to films having initially a dense silica matrix, characterized by a high visible refractive index (1.543 as compared to 1.458 for fused silica). Second, we show that the strong relaxation of the silica network (characterized by the decrease of the silica matrix refractive index from 1.543 to 1.475), due to the water penetration in the pores, is accompanied by a decrease of the pore volume fraction (from ∼30% before venting down to ∼15% after air exposure). © 2000 American Institute of Physics.Journal of Applied Physics 05/2000; 87(10):7303-7309. · 2.17 Impact Factor -
Article: Infrared ellipsometry study of evaporated SiO2 films: Matrix densification, porosity, water sorption
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ABSTRACT: The dielectric function of SiO2 films deposited by electron-beam evaporation on silicon substrates is determined by infrared ellipsometry between 700 and 5000 cm−1 one month after venting. Analysis of the absorption band associated with the Si–O–Si stretching mode at about 1075 cm−1 and of the water absorption band at about 3300 cm−1 enables the complex structure of the films, modeled as a densified silica matrix with pores partially or fully filled by water, to be characterized. The film density and visible refractive index values, which are estimated from the infrared parameters, are compared with the measured ones. A quantitative analysis of the wide water band, in which the contributions of three vibration modes (H–OH bonds, isolated Si–OH bonds, and neighboring Si–OH bonds) are clearly identified, shows that the Si–OH bond contribution is equal to 50% of the H–OH bond. This value is independent of the deposition parameters and seems to be representative of a particular type of porosity present in porous dielectric thin films. © 1997 American Institute of Physics.Journal of Applied Physics 07/1997; 82(3):1330-1335. · 2.17 Impact Factor -
Article: Determination of refractive-index profiles by a combination of visible and infrared ellipsometry measurements.
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ABSTRACT: Model inhomogeneous silicon oxynitride films were produced by ion-beam sputtering and characterized by ellipsometry in the visible and infrared ranges. These films exhibit strong intentional gradients of the refractive index that cannot be considered linear. A discrete description of the index profile with a few layers or a continuous description with a polynomial are examined; regressions by the use of measurements in the visible only or in the total spectral range (visible and infrared) are performed. Acquisition of data in an extended range is found to be a guarantee of the reliability of the calculated index profiles.Applied Optics 10/1996; 35(28):5540-4. · 1.41 Impact Factor -
Article: Infrared ellipsometry investigation of SiO(x)N(y) thin films on silicon.
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ABSTRACT: The dielectric function ˜ε (˜ε = ε(1) + iε(2)) of silicon oxynitride films deposited on silicon wafers by dual ion-beam sputtering is determined by infrared ellipsometry between 580 and 5000 cm(-1). The phase-separation model is unable to reproduce the experimental data. The dependence of ˜ε on stoichiometry is analyzed with the microscopic Si-centered tetrahedron model. The random-bonding model with five SiO(4-j)N(j) (j = 0-4) tetrahedra gives a good description of the spectra, provided the dielectric function of the mixed tetrahedra is carefully chosen.Applied Optics 09/1996; 35(25):4998-5004. · 1.41 Impact Factor -
Article: The initial stage in the formation of PtSi on Si(111) as followed by photoemission and spectroscopic ellipsometry
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ABSTRACT: The reaction of thin platinum layers with Si (111) to form platinum silicides was monitored in situ by core leve electron spectroscopy and spectroscopie ellipsometry. The chemical composition and stoichiometry of the reaction products were identified at crucial stages of the reaction via photoemission white the kinetics of the reaction could be followed via ellipsometry. Utilizing the optical constants of the reactants Pt and Si and the main reaction products PtSi and a Pt-rich PtxSi (x ~ 2) an initial room temperature reaction of Pt and Si extending some nanometres into the Pt overlayer has been detected and the evolution of the ellipsometric angles during the reaction of Pt and Si at elevated temperatures could be modeled quantitatively.Thin Solid Films 01/1995; 270. · 1.89 Impact Factor -
Article: Growth of low and high refractive index dielectric layers as studied by in situ ellipsometry
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ABSTRACT: Two materials, calcium fluoride and titanium dioxide, are used for this study of film growth. Ellipsometry measurements have been performed in situ on TiO2 films at fixed wavelength 2 during deposition, and on very thin CaF 2 films at fixed 2 or vs. 2 at different deposition steps. The value of the index of refraction n depends strongly on the nature and the temperature of the substrate and on the thickness of the deposit. For CaF 2 films deposited on Si wafers covered with their native oxide, a model including an interfacial layer, which takes into account the interaction of the film with the Si substrate, is proposed for reproducing the ellipsometric data. For CaF 2 films deposited on thick thermal SiO2, no evidence for chemical interaction has been found. For TiO2 films, it is shown that, even under a drastic control of the deposition parameters, reproducible samples are difficult to obtain with classical evaporation methods. The inhomogeneity of the index of refraction of TiO2 films is well described by a main "bulk" layer covered by a thin overlayer with a lower refractive index. The ellipsometry measurements correlate rather well with atomic force microscopy measurements. The influence of atmospheric moisture has been checked on TiO2 films.Thin Solid Films 01/1994; 253(257). · 1.89 Impact Factor -
Article: Temperature dependence of the dielectric function of silicon using in situ spectroscopic ellipsometry
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ABSTRACT: Silicon substrates are widely used for the optical study of transparent materials such as oxides, fluorides, etc., because of the large difference between the indices of refraction of the film and the substrate. Optimal conditions for growth of dielectric films require high substrate temperatures, and hence a good knowledge of the temperature dependence of the dielectric function of silicon. In the present study, the complex dielectric function of Si [111] was determined in the 1.5–4.7 eV spectral range from room temperature up to 450 °C, from measurements of the ellipsometric parameters tan ψ and cos Δ performed in ultra-high vacuum on silicon wafers covered with their native oxide, with a rotating polariser spectroscopic ellipsometer. A two-boundary model was used to account for the oxide layer. In the fundamental absorption spectral region, the major effect of temperature on the dielectric function is a shift and a broadening of the structures associated with critical points, in agreement with the literature. In the transparency region, the real part of the index of refraction is found to vary linearly with temperature for energies lower than ≈ 3 eV. Using this set of data as a reference, the temperature of a silicon substrate can be deduced from ellipsometric measurements in the 20–450 °C range with good accuracy (±3 °C).Thin Solid Films 01/1993; 233:166-170. · 1.89 Impact Factor -
Article: Study of CaF 2 growth on Si, a-SiO2 by in situ spectroscopic ellipsometry
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ABSTRACT: Ellipsometric measurements have been performed in situ on very thin CaF 2 films at fixed wavelength)o during deposition or vs. wavelength at different deposition steps. In modelling tan 7 j and cos A spectra as a function of 2 at different stages of growth, the Bruggeman effective medium theory is used to describe the interfaces. The value of the index of refraction n depends strongly on the nature and the temperature of the substrate, and on the thickness of CaF2 films. For films deposited at 400 'C on Si wafers covered with their native oxide, cross-sectional high resolution electron microscopy shows that, in spite of the presence of a SiO2 layer (2 nm thick), CaF2 grows on Si through this layer. A model including interfacial layers, which takes into account this interaction with the Si substrate, is proposed for reproducing ellipsometric data. For CaF2 films deposited on SiO2, no evidence for chemical interaction has been found.Thin Solid Films 01/1993; 233. · 1.89 Impact Factor -
Article: Estimate of the degree of inhomogeneity of the refractive index of dielectric films from spectroscopic ellipsometry.
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ABSTRACT: Dielectric thin films often present microstructures that give rise to a variation of the refractive index with the distance from the substrate. We propose a method of analysis of ellipsometric data for homogeneous and slightly inhomogeneous films that are deposited on transparent substrates. Assuming a linear refractive-index gradient, we are able to determine not only the average index and the thickness but also the degree of inhomogeneity of the films by spectroscopic ellipsometry at variable angles of incidence. We apply this method to titanium dioxide films deposited on glass, which present different degrees of inhomogeneity depending on the preparation conditions.Applied Optics 10/1992; 31(28):6056-61. · 1.41 Impact Factor -
Article: Determination of thickness and optical constants of thin films from photometric and ellipsometric measurements.
Applied Optics 01/1987; 25(24):4557. · 1.41 Impact Factor
Top co-authors
Top Journals
Institutions
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1997–2012
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Université Pierre et Marie Curie Paris 6
- Institut des nanosciences de Paris (INSP)
Paris, Ile-de-France, France
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2008
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Southern Taiwan University of Technology
Tainan, Taiwan, Taiwan -
French National Centre for Scientific Research
Lyon, Rhone-Alpes, France
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1993
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Université de Reims Champagne-Ardennes
Reims, Champagne-Ardenne, France
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