M. Froment

Phase formation processes in solution at the atomic level: Metal chalcogenide semiconductors

The improvement of deposition processes from solutions need a better understanding of the phase formation processes at the atomic level. This study concerns chemical bath deposition of chalcogenide semiconductors (CdS, ZnS, CdSe). Correlations between the growth mechanism and the structure of the films are established from Transmission Electron Microscopy (TEM) and High Resolution TEM measurements. Two dominant mechanisms are evidenced from structural studies, an atom by atom process for CdS and a cluster by cluster process for ZnS and CdSe. Structural defects in CdS are identified. Possible relations with optical properties and composition of the layers are given.

The thermal oxidation of silicon the special case of the growth of very thin films

Abstract The thermal oxidation of silicon is generally modelled by Deal and Groves theory based on the assumption that the oxygen molecules dissolve in silicon in interstitial positions and migrate to the Si-SiO2 interface where they react with the silicon substrate. Experimental results for oxidation in dry oxygen agree with this theory only for thick oxide films. The growth of very thin oxide films exhibits particular features which are discussed in this paper. For these films, the growth mechanism is different from that of thick films; this difference is possibly associated with the transport of oxygen atoms through the silica network. The effect of hydrogenated impurities is also discussed.

NICKEL PLATING BY PULSE ELECTROLYSIS : TEXTURAL AND MICROSTRUCTURAL MODIFICATIONS DUE TO ADSORPTION/DESORPTION PHENOMENA

Nickel growth from an organic-free Watts bath working under d.c. plating conditions is governed by several interfacial inhibitors such as H2, Hads or Ni(OH)2. These inhibitors determine most of the structural or macroscopic properties of the nickel plates. Pulse electrolysis (p.e.) is thus a powerful means of perturbing the adsorption-desorption phenomena occurring at the nickel/electrolyte interface and hence offers an opportunity of preparing deposits exhibiting better properties. Through an analysis of the textural and microstructural changes produced by p.e., we show that molecular inhibitors desorb during the relaxation time, while conversely other inhibitors like Hads or anions are more strongly adsorbed and inhibit the nickel cathodic process.

Behavior of Secondary Lithium and Aluminum‐Lithium Electrodes in Propylene Carbonate

The improvement of the lithium cyclic efficiency obtained by substituting an aluminum substrate for a lithium substrate is explained by means of morphological and kinetic studies of the electrodes in the molar solution LiClO/sub 4/- propylene carbonate. SEM observations show that the insertion rate of the lithium deposit into aluminum can be sufficiently high to avoid the dendritic growth; ESCA analysis also reveals that the chemical decomposition of propylene carbonate to form a polymeric membrane is reduced on the electrode surface. Electrochemical impedance measurements associated with polarization curves data point out that most of the surface is active giving rise to an exchange current density of about 17 ma/cm/sup 2/. These studies also demonstrate that diffusion processes in the passivating layer and in the bulk of the electrode are responsible for the limited lithium cycling performances with the aluminum substrate. 18 refs.

Nickel electrocrystallization—from nucleation to textures

TEM examinations of cross-sectioned Ni electrodeposits grown on oriented substrates show that their structure results of a competition between an epitaxial growth process and a non-epitaxial growth initiated by a substrate-independent nucleation. This independent nucleation is a necessary step prior to the progressive development of a definite fibre texture in thicker deposits. Both processes have been investigated separately on two kinds of cathodic substrates (either single crystals or amorphous carbon) by means of a permanent correlation between structural examinations and transients analysis. Results show that for an amorphous substrate multitwinned particles with a roughly hemispherical shape are generated by independent nucleation. Models are given which account well for experimental current—time transients. The situation is more complex when the substrate is a low-index plane of a single crystal (Cu or Ni) in which case several competitive growth processes are contribution to the whole current.

Analysis of the effects of substrate temperature, concentration of tin chloride and nature of dopants on the structural and electrical properties of sprayed SnO2 films

The correlations between structural and electrical properties of sprayed SnO2 films have been investigated as a function of substrate temperature (380–560 °C), concentration of tin precursor (0.02–0.8 M SnCl4) and the nature of the doping agent (chlorine, fluorine, antimony). High-resolution transmission electron microscopy has shown that chlorine or fluorine incorporation promotes the same type of defects, which are <0 1 1> twins. These latter behave as neutral defects, the density of which limits the carrier mobility of degenerated fluorine- or chlorine-doped films to around 20 cm2 V−1 s−1. The situation is totally different with antimony. Below the solubility limit in the SnO2 lattice (3%–4% Sb/Sn), Sn4+ are substituted by Sb5+, creating two conduction electrons per site and acting as point-charged defects which lower carrier mobility. Above this limit, the Sb3+ and Sb5+ forms coexist and are associated with an extremely large concentration of structural defects, especially twins induced by the Sb3+ species. These ions enter two-dimensional arrangements on both sides of the twins, making them planar charged defects.

Structural, electrical and interfacial properties of sprayed SnO2 films

Abstract The resistivity of SnO 2 films fabricated by spray pyrolysis of SnCl 4 methanolic solutions can vary over nearly six orders of magnitude depending on the conditions of preparation [substrate temperature, concentration of SnCl 4 and nature of the dopant (Cl, F, Sb)]. At low carrier density, the resistivity is determined by charge trapping at grain boundaries. In the case of highly degenerated materials, grain boundaries do not play a role anymore. The film resistivity is fully controlled by the bulk grain electrical properties, which are found to be dependent on the defect structure generated by the dopant. Thanks to Transmission Electron Microscopy, it is shown that chlorine or fluorine incorporation promotes the formation of the same neutral defects, which are {011} cassiterite twins. On the other hand, antimony gives rise to specific charged structural defects which strongly lower the carrier mobility. The electrical transport properties of F-doped SnO 2 films are better than those of Sb-doped materials. Considering the interfacial electron transfer with a redox system. Sb-doped electrodes are the most efficient because they achieve the highest carrier density, with performances approaching those of massive metal electrodes.

On the optical indices of oxide films as a function of their crystallization: Application to anodic TiO2 (anatase)

Abstract A statistical analysis of spectroreflectometric measurements on metallic samples covered with oxide films of different thicknesses allows the computation of the optical indices n and k (or dielectric constants ϵ 1 and ϵ 2 ) of the films. This method, which does not require a preliminary knowledge of the indices of the metallic substrate, has been applied to TiO 2 films grown anodically on titanium. It is known from electron diffraction studies that oxidation at high voltages (50–100 V) leads to the formation of crystalline anatase, whereas oxidation at low voltages (0–50 V) leads to films with a very short range structural order. The general features of the e 1 and e 2 variations between 0.62 and 4.96 eV are similar to those of an insulator, and they can be approximated in terms of Lorentzian oscillators. The differences arising in the optical spectra, particularly around the fundamental absorption edge, as a function of the crystallographic structure of the films are indicated.

High-resolution transmission electron microscopy study of chemically deposited cadmium sulphide thin films from aqueous ammonia solutions

Abstract Cadmium sulphide thin films have been deposited chemically at near room temperature in aqueous ammonia solutions using thiourea as the sulphur source. The composition of the solution was [CdSO4] = 1.4 × 10−2M, [Thiourea] = 2.8 × 10−2 M, [NH3] = 1.74 M. The structure of the layers and the growth mechanism are studied by using high resolution transmission electron microscopy (HRTEM). The layers are formed of well defined crystallites (few tens of nm size) with only the hexagonal structure, whereas the colloids formed in solution by the parallel homogeneous reaction are composed of much smaller crystallites (3-6 nm) and present a mixed hexagonal/cubic structure. This is related to the polymorphic tendency of the CdS structure, and indicates a strong influence of local parameters in the deposition conditions. The results also indicate that the growth more likely proceeds via an atomie mechanism, rather than a cluster coagulation mechanism. HRTEM images display well defined atomie projections for crys...

A kinetical model for the electrochemical grooving of grain boundaries

Abstract In the case of FeNiCr alloys (without precipitation), we propose an electrochemical method which allows us to obtain a reproducible selective corrosion of grain boundaries (H 2 SO 4 -2N-25°C in the transpassive domain). We establish a model of dissolution to explain the morphology of etched grooves. From this model we deduce a parameter α 0 which characterizes the intergranular corrosion susceptibility of a given alloy. Electrochemical kinetic allows us to calculate the ratio of the density of active sites at the emergence of the grain boundary and on the surface of the grain. This electrochemical method permits us to study the effects of the segregation phenomena, of composition alloy and of grain boundary structure on intergranular corrosion.