Elaboration et caracterisation de monocristaux de V2O3 par transport en phase vapeur

Single crystals of V₂O₃ are obtained by HCl or TeCl₄ chemical transport method in sealed quartz tubes with temperature gradients of 1050–930°C and 990–900°C respectively. The hexagonal prims are 5 mm long and 1 or 2 mm wide. Both kinds of crystals are characterized by sharp insulator-metal transition at 156 and 137 K respectively, they present a large hysteresis. The sharp decrease in resistivity with increasing temperature is of the order of 10⁸ at the transition.     

Influence du dopage p et n sur la croissance en phase vapeur de germanium sur substrats germanium

Epitaxial layers of n- and p-doped germanium were grown on a germanium substrate by chemical vapour transport in an open flow system using a disproportionation reaction involving GeI₂. n- and p-doped germanium were obtained by incorporation of arsenic and boron. Growth was in the temperature range 320–380°C at a rate of the order of 0,2-1,4 μm h⁻¹. The epitaxial process was found to be sensitive not only to substrate temperature but also to pressure and gas flow rates of dopant species.     

Structural and optical properties of sintered ZnSxSe1−x films

Sintered ZnS_xSe_1−x films have been prepared in the entire composition range from ZnSe to ZnS by using the screen printing method. To deposit good quality films, optimum conditions have been determined. Wide band gap ternary films have wide applications in solar cells. The band gap of these films are determined by reflection spectra in the wavelength range of 325–600 nm using the Tauc relation. These films have a direct band gap, which varies from 3.50 eV for ZnS to 2.66 eV for ZnSe films. The wurtzite structure of ZnSSe films was confirmed by X-ray diffraction analysis of these films.     

Study of optical constants in CdxSe1−x

Thin films of Cd_xSe_1−x (0<x<1) have been deposited by the vacuum evaporation technique onto ultra clean substrates maintained at 300°C. The optical properties (specially transmission spectra) of these films were studied. The wavelength dependence of refractive index and extinction coefficient of the films along with the variation of these constants with composition was studied.     

Crystallization kinetics of SbxSe100−x thin films

The crystallization kinetics in Sb_xSe_100−x films with 39≤x≤58is studied by monitoring the optical transmission of the films during both isothermal and constant rate heatings. The structure of the films upon crystallization and at certain intermediate stages is studied by electron microscopy techniques. The results are analyzed in the frame of the Johnson-Mehl-Avrami theory in order to determine the kinetic parameters (Avrami exponent, activation energy and frequency factor) in addition to the crystallization temperature. The results show that film crystallization is always preceeded by a relaxation process which modifies substantially the optical properties of the amorphous material. Amorphous films with compositions close to the stoichiometric compound (Sb₂Se₃) are found to show the highest activation energy for crystallization.     

Effet de la structure sur les propriétés optiques de couches minces de composés de tungstène obtenues par décomposition en phase vapeur: Influence sur la sélectivité spectrale

Black or reflective tungsten thin films were deposited at 400°C onto fused quartz substrates by chemical vapour deposition (CVD) of tungsten hexacarbonyl in argon at atmospheric pressure. The structure was investigated by X-ray and electronic diffraction.

This analysis shows that the structure and composition of thin films depend on the preparation conditions and annealing. Optical properties and selectivity characteristics are also presented.     

Étude de la cinétique de croissance de couches ZnSe ET ZnS épitaxiées sur fluorine par transport en phase vapeur

Epitaxial layers of ZnSe and ZnS were grown on (111) calcium fluoride (CaF₂) substrates in an open tube system by reaction of hydrogen with the compounds in powdered form. Typical growth rates of 1–2 μm h^?1 for ZnSe and 3–5 μm h^?1 for ZnS have been obtained. The parameters investigated were source temperature, substrate temperature and hydrogen flow rate.The experimental results were compared with theoretical growth rates calculated from the partial pressures of the components. In this case, the growth rate is proportional to the concentration difference between the source and the substrate and contains one adjustable parameter which depends on the substrate temperature and vapour phase composition.     

Etude thermochimique de la synthese en phase vapeur de Y3Fe5O12

Thermochemistry of oxidation and hydrolysis of YCl₃ and FeCl₂ have been studied. It has been shown that Y₃Fe₅O₁₂ cannot be formed by hydrolysis. Conditions of synthesis have been established in oxidation, particularly it is pointed out that excess of HCl or Cl₂ is not recommended for moderating reactions. It makes the synthesis difficult to control. Theoretical results are compared to experimental data.     

Composition dependence of optical constants of Ge1 − xSe2Pbx thin films

Optical constants of vacuum-evaporated thin films in the Ge_1 − xSe₂Pb_x (x = 0, 0.2, 0.4, 0.6) system were calculated from reflectance and transmittance spectra. It is found that the films exhibit a non-direct gap, which decreases with increasing Pb content. The variation in the refractive index and the imaginary part of the dielectric constant with photon energy is reported. The relationship between the optical gap and chemical composition in chalcogenide glasses is discussed in terms of the average heat of atomization.     

Optical transitions near the band edge in bulk CuInxGa1−xSe2 from ellipsometric measurements

From the analysis of the variation of optical absorption coefficient with incident photon energy between 0.8 and 2.6 eV, obtained from ellipsometric data, the energy E_G of the fundamental absorption edge and E_G′ of the forbidden direct transition for CuIn_xGa_1−xSe₂ alloys are estimated. The change in E_G and the spin-orbit splitting Δ_SO=E_G′−E_G with the composition x can be represented by parabolic expression of the form E_G(x)=E_G(0)+ax+bx² and Δ_SO(x)=Δ_SO(0)+a′x+b′x², respectively. b and b′ are called “bowing parameters”. Theoretical fit gives a=0.875 eV, b=0.198 eV, a′=0.341 eV and b′=−0.431 eV. The positive sign of b and negative sign of b′ are in agreement with the theoretical prediction of Wei and Zunger [Phys. Rev. B 39 (1989) 6279].     

Optimisation de la preparation de films minces d'oxyde d'etain par transport en phase gazeuse

Nesa coating is emphasized as a constitutive element of low cost solar cell converters. Gas phase transport method is used; the fabrication parameters are independently varied in order to describe their influence and reach optimized specifications.     

Preparation par transport en phase vapeur et caracterisation de monocristaux d' oxyde ternaire V1−xNbxOw(0 ⩽ x ⩽ 0,33)

Single crystals of V_1?xNb_xO₂ ternary oxide (0 ? x ? 0,33) are prepared by chemical transport using TeCl₄. The compositions are determined by thermogravimétric analysis under oxygen, X-ray diffraction measurements and evaluation of the insulator-metal transition temperature.