G. Couturier

Photoactivity enhancement of WS2 sputtered thin films by use of nickel

Abstract Photoactive thin films of WS2 have been prepared by radio frequency magnetron sputtering. Physico-chemical properties of the films are mainly investigated by Rutherford backscattering and X-ray diffraction. Optical and transport properties are also studied. Films sputtered at a high substrate temperature are made of small bidimensional grains (2H–WS2) surrounded by amorphous boundaries. The photoactivity of these films is weak because of the electron trapping at boundaries. Films sputtered at a low substrate temperature are amorphous and then annealed to produce bidimensional films. It is shown that annealed Ni-coated films give highly textured samples. A nickel–sulphur phase is assumed to act as a surfactant. The strong photoactivity enhancement of these films is attributed to an increase in the grain size.

Influence of oxygen on electrical properties of βPbF2 thin films

Abstract In this paper, a study of oxygen adsorption by βPbF 2 thin films is presented. In a first part, results of oxygen analysis by nuclear methods and by ESCA are presented. Then, the influence of oxygen on both interfacial and bulk electrical properties is studied. Besides a slow diffusion process in the film, the results seem to show the presence of a thin oxidized layer at the surface.

A.c. study of metal-β-PbF2-metal thin film structures

Abstract The electrical properties of the ionic conductor β-PbF 2 were studied using admittance and impedance diagrams. Measurements were made on symmetrical metal-β-PbF 2 -metal thin film structures which were obtained by thermal evaporation in vacuum. It was possible to distinguish between bulk and interfacial properties. The conductivity of the PbF 2 films is compared with that obtained for bulk samples. A description of the interfacial layer between the electrode and the solid electrolyte is given.

Preparation and characterization of gallium(III) fluoride thin films

Abstract This paper deals with the preparation of initial powders and thin films of gallium fluoride. The crystallographic state and the composition of thin films as a function of their preparation conditions were analysed by X-ray diffraction, backscattering of α particles and photoelectron spectroscopy. The band gap and the transport properties were deduced from optical absorption measurements and admittance spectroscopy.

A study of the Au/WSe2 interface

Abstract Comparing the work functions of gold an n-type tungsten selenide a blocking contact is normally expected at the Au/n-type WSe 2 interface. If a Schottky contact is actually obtained when the gold electrode is deposited on an oxidized WSe 2 surface, it is not usually the case for a freshly cleaved surface: the resulting contact is generally ohmic. We have studied the influence of a controlled oxide layer on the characteristics of this interface. Using both results obtained in this study and those concerning the polyiodide solution/WSe 2 interface it is possible to improve our understanding of the WSe 2 surface properties and to explain the observed anomalous ohmic behavior of the interface with gold. It is finally shown that the interfacial oxide layer can be optimized to improve the efficiency of the photovoltaic cell using this interface (we have obtained a 8.6% yield).

Fluorine-gas treatment of InP(100): physico-chemical characterization of the obtained components

Abstract Fluoride thin film InP (100) structures have been prepared by oxidization under fluorine gas for different temperatures and times. The composition of the obtained layers has been studied using various analytical techniques (Rutherford backscattering, infrared absorption and XPS). The bulk composition of the obtained layers is InF3. Whatever the fluorination temperature (TF) the formed phosphorus fluoride is eliminated during the reaction. An XPS study has shown that at the fluoride-InP interface, phosphorus, like indium, is chemically bound to fluorine. However it is demonstrated that for TF ⩽ 200 °C an important oxygen content, corresponding to absorbed water, is observed in the bulk of the fluoride thin films; on the contrary, for TF = 300 ° C, the grown InF3 layers are insensitive to an ageing in moist air.

Electronic and ionic conductivity measurements in ionic conductors with a high band gap

Abstract The classical models of mixed conductors study always consider an electronic conductivity through the conduction (or valence) band. These models are inadequate to interpret the behaviour of materials with high band gap like fluorine compounds where the electronic conductivity is supported by localized states in the gap. A model, which takes into account the very low mobility of electrons, is proposed to explain the current variation versus time. Finally a new method of ionic conductivity measurement is explained.

Electrical behaviour of epitaxial SrF2/InP(100) diodes

Abstract In this paper, it is shown that the conductivity of SrF 2 layers epitaxially grown onto n-type InP(100) substrates is about 10 −16 S cm −1 at room temperature with an activation energy close to 0.7 eV. An ionic contribution only appears for temperatures higher than 380 K. The electrical properties of MIS structures (metal/SrF 2 /n-type InP(100)) were investigated by means of capacitance-voltage characteristics ( C(V) ) and admittance spectroscopy measurements. The results strongly depend on the cleaning process applied to the semiconductor surface. The classical cleaning process (H 2 SO 4 · H 2 O 2 · H 2 O + HFdeoxidization) did not allow the accumulation regime to be reached in the semiconductor. In this case the trap density is greater than 10 12 eV −1 cm −2 . On the other hand, it was demonstrated that treatment with ammonium sulphide improves the qualities of the device. In particular, the density of states can be reduced to 10 11 eV −1 cm −2 and the accumulation regime is achieved.

Indium fluoride thin films prepared by sublimation under vacuum

Thin films of indium fluoride were obtained by sublimation under vacuum of high purity powders. Their crystallographic state and composition, as a function of the preparation conditions, were investigated by X-ray diffraction, Rutherford backscattering of α particles, mass spectrometry and photoelectron spectroscopy. The energy band gap of the obtained layers and their electrical characteristics were respectively deduced from optical absorption measurements and admittance spectroscopy.

A study of thermally stimulated currents in thin films of a good ionic conductor (βPbF2) with blocking electrodes

Abstract Thin film samples of βPbF2 are studied by stimulated currents, the TSC peak of lower temperature, independent of the polarization voltage is attributed to the space charge depolarization. On the basis of simplifying hypothesis an exact analytical solution of this problem is also proposed; the comparison with the experimental results allows to get a value of the activation energy of mobile species and to estimate the thickness of the space charge. Electron transfers at the electrodes are assumed to explain the second peak.