A. Donnadieu

Study on the optical and electrochromic properties of polycrystalline WO3 thin films prepared by CVD

Abstract Polycrystalline WO 3 thin films were prepared by annealing, under various conditions, of black or reflective tungsten layers produced by CVD on fused quartz or SnO 2 coated substrates. The optical gap was investigated and found to depend on film thickness. It varies ∼2.5 eV for the thicker films to 3.1 eV for the thinner films. The values of the electrochromic efficiency, using protons as inserted ions, varies between 30 and 40 cm 2 /C for all kinds of films and depends on the film preparation and the coloring-bleaching cycle. Coloring and bleaching times and response of the films are also found to be enhanced with cycling and influenced by the preparation conditions.

The influence of surface texture on the solar absorptance of black molybdenum

Abstract The efficiency of photothermal solar energy conversion increases with the solar absorptance of the converter surface. The absorptance of solar materials can be optimized by employing combinations of intrinsic optical properties, metal-oxide composites, antireflection and surface texture. Previous work has shown that the optical properties of MoO2 and molybdenum, the dispersion of the metal in the oxide and antireflection with Si3N4 all enhance the solar absorptance of black molybdenum. In this paper we prove that surface texture can increase the absorptance of black molybdenum by 18% without increasing the thermal emittance. The magnitude of the increase depends on substrate roughness and film thickness. Black molybdenum films exhibiting these optical properties were chemically vapor deposited from Mo(CO)6, and were neither annealed nor antireflected.

Refractive index dependence on optical gap in amorphous silicon—Part I. Si prepared by glow discharge

Abstract We present here our studies concerning the variation of refractive index with the optical gap in amorphous silicon prepared by glow discharge decomposition of silane. The study has been carried out in the light of the models of Penn, Wemple-Didomenico, Ravindra et al ., Moss and Bahl-Bhagat. It is essentially seen that the model of Bahl-Bhagat is good enough to explain the relative shifts in the refractive indices in terms of the changes in the gaps on introduction of hydrogen into amorphous silicon. However, because of weaknesses associated with the fitting parameters, we propose here an alternate model which explains fairly well the dependence of the index of refraction on the optical gap. Furthermore, to explain the gradient of the refractive index vs optical gap plots, we see that a simple model like that of Moss would suffice. This is all the more interesting by virtue of the fact that the Moss formula is basically representative of the atomic picture. Of course, the constant depends on conditions during formation of the sample. Under some limiting conditions, the Bahl-Bhagat relation is shown to reduce to the linear form like that of Ravindra et al . We also attempt to analyse qualitatively the dependence of dispersion energy and the average excitation energy on temperature in the light of the Wemple-Didomenico model. The present study has been carried out for samples prepared by glow discharge at different substrate temperatures and with different hydrogen concentrations.

Electrochromic effect in WO3 thin films prepared by CVD

Abstract Polycrystalline WO 3 thin films were produced by CVD on fused quartz or SnO 2 coated pyrex substrates. The film structures were determined. The electrochromic phenomenon was observed in this kind of films in a two electrode electrochemical cell using protons as inserting ions. Electrochromic performances were investigated using a three electrode electrochemical cell in two different electrolytes.

The effects of surface characteristics on the visible and uv optical properties of CVD amorphous silicon films

Silicon films were deposited by the pyrolytic decomposition of silane onto fused quartz substrates held at various temperatures in the range 550 to 800°C. The films are amorphous when prepared below a substrate temperature Ts of 650 to 700°C. Scanning electron micrographs indicated that surface roughness increased with increasing Ts. Optical properties were determined from near-normal specular reflectance and transmittance measurements in the energy range 0.5 to 14 eV. Diffuse reflectance measurements in the energy range 0.5 to 5.5 eV permitted the determination of the hemispherical reflectance. These measurements indicated that the CVD amorphous silicon films significantly scattered photons having energies above 2 eV. In addition, the uv measurements indicated the presence of silicon dioxide layers which was confirmed by specular reflectance measurements of samples annealed in air. The presence of scattering through surface roughness and the influence of oxide layers must be taken into account carefully when optical measurements above 2 eV are made the basis for a subsequent Kramers-Kronig analysis. Theoretical conclusions based on the results of such an analysis deserve confidence only if the surface roughness and oxide layers do not detectably affect the specularity of the optical measurements.

Electrochromism in polycrystalline WO3 thin films prepared by chemical vapour deposition at high temperature

Abstract Polycrystalline WO3 thin films were prepared by annealing, under various conditions, black or reflective tungsten layers produced by chemical vapour deposition on fused quartz or SnO2-coated substrates. The electrochromic properties of these films (colouring and bleaching times, injected charge optical density) were investigated in two- and three-electrode electrochemical cells. An improvement in the above properties with increasing film thickness was observed. The results obtained were discussed on the basis of an assumed fast electronic and a slower ionic injection. A preliminary study of the electrocoloration at various frequencies supports the above idea.

The effect of film thickness on the reflectivity of glow-discharge amorphous silicon

Abstract Diffuse and hemispherical reflectance measurements in the energy range 0.5 to 5.5 eV have been performed on glow-discharge amorphous silicon films. The spectra depend on both the deposition temperature and the thickness of the films. These measurements indicate that photons of energies above 2 eV are scattered by films with a thickness larger than 1 μm. The presence of scattering through surface roughness must be taken into account when optical measurements above 2 eV are made as basis for calculations.

Temperature dependent studies of the optical properties of post-hydrogenated sputtered a-Si:H

Abstract In the present study, post-hydrogenation and annealing effects on the optical properties of amorphous silicon films deposited by rf sputtering are described. It is essentially seen that large changes in the hydrogen concentration on the various sites occur and the optical properties appear to be strongly dependent on the tissue. A decrease of the residual disorder with post-hydrogenation for hydrogen saturated or unsaturated samples is deduced from the determination of the structural part Fs of the Urbach tail.

Variations of optical properties between 95 and 723 K of boron doped a:SiH films prepared by glow discharge

Abstract The study of optical properties of boron doped a:SiH films shows that the boron atoms disturb the silicon matrix. Contrary to hydrogen, boron doping makes the optical gap decrease. When the measurement temperature T m becomes higher than the preparation temperature T s , irreversible variations of optical gap appear in a sense opposite to those of undoped samples. This study leads us to believe that boron doping creates an increase in the density of states in the forbidden band.

Boron doping effect on optical properties of a-Si films prepared by CVD: Post-hydrogenated and H-exodiffusion study

Abstract The effect of post-hydrogenation and H-exodiffusion on the optical parameters is studied in boron doped amorphous silicon prepared by CVD. The optical parameters, which depend on boron concentration, vary when hydrogen is incorporated. The variations observed are compatible with a decreasing of the spin density. When the measurement temperature increases, irreversible effects appear due to evolution of hydrogen. These results are interpreted by a simple model of electronic compensation of defects by doping and a variation of the density of electrically active boron atoms with hydrogen concentration.