Francois Flory

Scattering from multilayer thin films: theory and experiment

A theoretical method is described for calculating the angular scattering curve for any given thin-film multilayer in which the various surfaces and interfaces are assumed to be rough. The energy that is scattered in any particular direction depends on two factors. The first is solely a function of the characteristics of the ideal multilayer and the observation conditions. The second depends on the roughnesses of the various surfaces, that is, on their autocorrelation and cross-correlation functions. The expressions that are obtained are completely general because no restrictive hypothesis has been developed on the relationship that may exist between the roughnesses of the various interfaces. The principal features of the apparatus used for the scattering measurements are briefly described. Next, several results of calculation and measurement are given as illustrations. The measurements show that polished surfaces of high optical quality often show marked anisotropy.

Morphology and structure of TiO2 thin layers vs. thickness and substrate temperature

Abstract TiO 2 thin layers are often used in multilayer coatings for optical filters and this makes their study especially interesting. Different TiO 2 thin layers corresponding to different thicknesses and deposited at different temperatures have been prepared by vapour deposition onto amorphous substrates. Most of these layers are amorphous but small crystals of anatase and rutile can also exist. These crystals are more frequent (i) if the substrate temperature is raised (350 °C) and/or (ii) if the thickness of the thin layer is higher. The surface roughness of TiO 2 thin layers has also been investigated by electron microscopy by two different and complementary methods: direct imaging by transmission electron microscopy and carbon-platinum replica observation. The thin layer surfaces are rather irregular, and this practically independently of the film thickness and of the substrate temperature during the deposition. Surface irregularities have been measured to reach about 10 nm. They have been shown to be slightly reduced on increasing the thin layer thicknesses from 35 to 70 nm. Each layer is made of blocks about 40 nm in size and each block is made of smaller blocks (about ten times smaller). The origin of this phenomenon has been sought. The formation of the smaller blocks is in agreement with existing theories of nucleation and growth of amorphous thin films. An attempt to find the origin of the larger blocks has also been carried out. This could be related to the permanent chemical transformation occuring during the thin layer formation. Finally, we relate the optical properties of the TiO 2 thin layers to their morphology and structure.

m-lines technique: prism coupling measurement and discussion of accuracy for homogeneous waveguides

A method is proposed to measure the thickness of the air layer between the prism and the waveguide in a totally reflecting prism coupler. The coupling efficiency of a Gaussian beam from the prism into the waveguide can be calculated when the air-layer thickness (ALT) is known. To perform measurements of the indices and thicknesses of planar waveguides using the m-lines technique, it is necessary to have a good knowledge of the prisms characteristics and to accurately measure the angles. However, we show by means of an example that the small distance between the prism and the guide (i.e. the ALT) should be taken into account in order to achieve accurate measurements.

Refractive index and inhomogeneity of thin films

The fact that the optical characteristics of thin-film materials are generally different from those of the same materials in bulk form is well known. The differences depend very much on the conditions in which the deposition has been carried out. A good understanding of these differences, their causes, and the influence of deposition parameters is vital if we are to be able to improve coating quality. We have developed two complementary methods with the objective of deriving information on the index of refraction and its variation throughout the thickness of the film. Perceptible optical inhomogeneity is normally present and appreciable inhomogeneity is frequently present in thin films. Such inhomogeneity is usually associated with layer microstructure. The first is a postdeposition technique that makes use of measurements in air of the transmittance and reflectance of the layer under study over a wide wavelength region. The second, in contrast, makes use of in situ measurements, that is measurements made under vacuum and during the actual deposition of the layer. We shall show with the help of several examples that the two methods lead to results that are consistent and demonstrate the existence in deposited materials of an inherent variation of the index of refraction normal to the surface. The thermal sensitivity of the layer properties and their tendency to adsorb atmospheric moisture must be taken into account before the residual differences between the two techniques can be explained.

Effect of the thickness of the MoO3 layers on optical properties of MoO3/Ag/MoO3 multilayer structures

The electrical and optical properties of MoO3/Ag/MoO3 multilayer structures have been studied using the Ag deposition rate and layer thicknesses as parameters. When the silver film is deposited at 0.20u2009nm/s rate, the silver layer thickness necessary to achieve the percolation threshold of the resistivity ρ towards conductive structures is 10u2009nm. Below 10u2009nm, the films are semiconductor and above the films are conductors. In the present work, the variation of the thicknesses of top and bottom MoO3 layers is shown to strongly modify the optical properties of the multilayer structures. By using a Ag thickness of 10u2009nm, we demonstrate an increasing of the transmittance of the MoO3/Ag/MoO3 structures by optimizing the MoO3 layers thicknesses. When the MoO3 bottom layer is 20u2009nm thick, and the MoO3 top layer is 35u2009nm, the maximum transmission is 86% at the wavelength of 465u2009nm, while the averaged transmission in the visible range (350u2009nm-800u2009nm) is 70%. The best measured conductivity, σu2009=u20091.1u2009×u2009105 (Ω cm)-1, corresponds also to this MoO3 (20u2009nm)/Ag (10u2009nm)/MoO3 (35u2009nm) structure. A good qualitative agreement between the theoretical calculations of the variation of the optical transmittance and reflectance of the MoO3/Ag/MoO3 structures is also highlighted.

Anisotropy in thin films: modeling and measurement of guided and nonguided optical properties: application to TiO 2 films

The main purpose of this research is to study the anisotropic behavior of dielectric material in thin-film form. First we present a theory based on a 4 × 4 transfer matrix linking tangential components of the electromagnetic field on one interface to the tangential components of the electromagnetic field on the other interface of an anisotropic thin film. A biaxial model is associated with the columnar structure of the layer. The comparison between measurements of the transmission in normal incidence in cross-polarized light and of guided-mode propagation constants with the calculations allows us to study the biaxial behavior of TiO(2) films. The excellent consistency between measurements and computations demonstrates the validity of the model based on the columnar structure.

Optical modeling of organic solar cells based on CuPc and C60.

We have investigated the influence of the poly(3,4-ethylenedioxythiophene)-blend-poly(styrene-sulfonate) (PEDOT:PSS) layer on the short-circuit current density (J(sc)) of single planar heterojunction organic solar cells based on a copper phthalocyanine (CuPc)-buckminsterfullerene (C(60)) active layer. Complete optical and electrical modeling of the cell has been performed taking into account optical interferences and exciton diffusion. Comparison of experimental and simulated external quantum efficiency has allowed us to estimate the exciton diffusion length to be 37 nm for the CuPc and 19 nm for the C(60). The dependence of short-circuit current densities versus the thickness of the PEDOT:PSS layer is analyzed and compared with experimental data. It is found that the variation in short-circuit current densities could be explained by optical interferences.

Optical characterization of thin films by guided waves

The study of guided waves in a thin layer allows a precise characterization of refractive index and thickness. Optical anisotropy can also be measured. We show how this technique can be applied to the characterization of a multilayer structure.

Nanostructured ZnO coatings grown by pulsed laser deposition for optical gas sensing of butane

We report the detection of 100ppm of butane in superatmospheric N2 or air with an m-lines setup. The sensing elements are ZnO-nanostructured coatings prepared by pulsed laser deposition. The deposition technique was optimized to obtain highly transparent films of 1cm2 in area and several hundreds nanometer thick. ZnO structures preserve gas sensitivity even when deposited at room temperature. Refractive index variations down to 0.005 were detected and typical variations of about 20% were induced during ZnO film-butane contact.

Fluorescence of Ta 2 O 5 thin films doped by kilo-electron-volt Er implantation: application to microcavities

Luminescent layers are prepared by the implantation of kilo-electron-volt Er ions into tantalum pentoxide (Ta(2)O(5)) thin films made by ion plating. The implantation fluences range from 3.3 × 10(14) to 2 × 10(15) ions/cm(2), and the energies range from 190 to 380 keV. Refractive index, extinction coefficient, and losses on guided propagation are investigated. We show that these Er-implanted layers present an absorption as low as that of the nonimplanted films. When optically pumped with an Ar(+) laser (λ = 0.488 μm) beam, implanted films show peaked fluorescence spectra centered near 1.53 and 0.532 μm. We show that the fluorescence intensity is correlated with the intensity of the pump beam in the region where Er ions are implanted. Radiation patterns of Er ions located inside a single layer or inside a Ta(2)O(5)/SiO(2) dielectric stack made by ion plating are also investigated. We show that, in any case, spontaneous emission of Er ions can be spatially controlled.